So when someone casts Shadow Blade - It summons a shadowy blade, etc., etc. 

However, when the spell rolls - it's only applying a damage roll. It's not behaving like a standard "weapon" - where it's rolling a "to hit."

Is there something I am missing?

Reread the discription.

It states "It deals a 2d8 psychic damage on a hit....." Which implies a to hit role is needed. Otherwise it would just say something like "It deals 2d8 psychic damage." period.

It's a wonky setup. The target is self, and the spell technically only causes the blade to appear, you still need to use your action to attack with it (though you can attack with it as a bonus action using two weapon fighting on subsequent turns). So the spell lists the damage dice. But the spell doesn't involve the attack action so there's no to-hit associated with it. Also, some argue that sword's damage should include your Dex or Strength mod, and the calculation as listed doesn't include that. Best advice for spells like this is to create a homebrew weapon or custom action that has the relevant properties, and just use the spell in the spell listing to track spell slots only.

There is no calculation listed just the shadow weapons damage 2d8 and damage type since the type is different from a normal weapon. It even states its treated like a finesse, light or thrown weapon.

Treat it just like any other weapon that cause damage. Add in your normal melee modifiers since it does not state you use your spell attack modifier.

Makes sense. Just odd - because on a character player sheet (rolling in D&D Beyond specifically) - the player would typically need an item to roll using (like a "dagger" for the attack or the thrown) - otherwise, they'd need to roll the D20 and manually add any bonuses and such.

I think the custom weapon is the way I am going to go and just have the Rogue Trickster add (sword and dagger/thrown) as "weapons" in their inventory to use - and just home brew the stuffs.

You don't need the spell to specify the 'to-hit' because it already says : "It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). ".

As detailed in Chapter 9: Combat, Making an Attack, Attack Rolls

Modifiers to the Roll

When a character makes an attack roll, the two most common modifiers to the roll are an ability modifier and the character’s proficiency bonus. When a monster makes an attack roll, it uses whatever modifier is provided in its stat block.

Ability Modifier. The ability modifier used for a melee weapon attack is Strength, and the ability modifier used for a ranged weapon attack is Dexterity. Weapons that have the finesse or thrown property break this rule.

Some spells also require an attack roll. The ability modifier used for a spell attack depends on the spellcasting ability of the spellcaster, as explained in chapter 10.

Proficiency Bonus. You add your proficiency bonus to your attack roll when you attack using a weapon with which you have proficiency, as well as when you attack with a spell.

As clarified in Chapter 9: Combat, Damage & Healing, Damage Rolls

"When attacking with a weapon, you add your ability modifier — the same modifier used for the attack roll — to the damage. A spell tells you which dice to roll for damage and whether to add any modifiers."

You roll the ability mod for damage for weapons. The spell tells you the sword counts as a weapon. So, yes, you add your Str / Dex to the damage roll. Just, instead of 1d8 piercing or 1d10 slashing etc, it does 2d8 psychic.

So, 

Attack Roll: 1d20 + Str/Dex mod + prof bonus

Damage Roll: 2d8 + Str/Dex mod psychic damage.

The problem that the OP is raising is that DDB doesn't automate the process, not that the OP doesn't understand how to do it.

 Not really a problem. A spell like this isn't going to automate because the spell just creates a temporary item. It cannot temporarily add that item to the sheet. This is why Custom Actions and Custom Items were added to the sheet. It's so you can just add it yourself if you need it. 

Decided to make a video. Coz why not? ¯\_(ツ)_/¯

I just added a weapon that fitted the criteria (simple, melee, finesse, etc.) and customised it with the name: "Shadow Blade - spell for damage".
Added the "Advantage on attack rolls in dim light or darkness." to the notes.

Gives me the correct roll, and points me to the spell list to roll the damage.

Quote from Kai_Varjo >>

I just added a weapon that fitted the criteria (simple, melee, finesse, etc.) and customised it with the name: "Shadow Blade - spell for damage".
Added the "Advantage on attack rolls in dim light or darkness." to the notes.

Gives me the correct roll, and points me to the spell list to roll the damage.

 
And how do you get it to do 2d8? No weapon has those with 2d8 damage and you cannot customise. I'd love to know how to do that, can you provide a guide please?

If you go to your spell list, Shadow Blade has the 2d8 there for rolling the damage.

I use the customised weapon for rolling the attack roll, then if it hits I go to my spells to roll the damage.

Quote from Kai_Varjo >>

If you go to your spell list, Shadow Blade has the 2d8 there for rolling the damage.

I use the customised weapon for rolling the attack roll, then if it hits I go to my spells to roll the damage.

 
I think I prefer my way, it's all here in one line that way. Also, the damage indicated in the spell for rolling is actually wrong, due to a sheet limitation. But you do you. ¯\_(ツ)_/¯

What do you mean the damage is wrong? 2d8 then scales as you upcast it....works fine on my sheet.

Quote from Kai_Varjo >>

What do you mean the damage is wrong? 2d8 then scales as you upcast it....works fine on my sheet.

 
It is supposed to add your Str/Dex bonus to damage because it is treated as a finesse melee weapon. The only reason the sheet doesn't is because the sheet isn't coded that way for spells. 

Nah, I read it as spell only damage. The finesse etc just applies to the roll to hit in my opinion.

To each their own though.

Quote from Kai_Varjo >>

Nah, I read it as spell only damage. The finesse etc just applies to the roll to hit in my opinion.

To each their own though.

 
My way was confirmed by the official D&D Rules Designer. But aye, to each their own game.

Quote from SirTawmis >>

So when someone casts Shadow Blade - It summons a shadowy blade, etc., etc. 

However, when the spell rolls - it's only applying a damage roll. It's not behaving like a standard "weapon" - where it's rolling a "to hit."

Is there something I am missing?

 

 It's a weapon made from the spell. The weapon damage is based on the spell, it's quiet simple, and I use it on my warlock-rogue builds. 

cast spell, have (insert flavor) shadowy swords in your hands you then treat it as a sword attack with a damage role based on the spell. As a rogue I still get the sneak attack, if the attack counts as a sneak attack. If you use it as a thrown weapon, it disperses at the end of the turn, but as a bonus action you can have it in your hands again on your next turn. In dark environments you get advantage on the attack which is good for the roguelock. 

((Note I usually use it with my offhand, and never throw it. That way my main hand can be a rapier improved hex weapon.)  

Quote from gothicshark >>

Quote from SirTawmis >>

So when someone casts Shadow Blade - It summons a shadowy blade, etc., etc. 

However, when the spell rolls - it's only applying a damage roll. It's not behaving like a standard "weapon" - where it's rolling a "to hit."

Is there something I am missing?

 

 It's a weapon made from the spell. The weapon damage is based on the spell, it's quiet simple, and I use it on my warlock-rogue builds. 

cast spell, have (insert flavor) shadowy swords in your hands you then treat it as a sword attack with a damage role based on the spell. As a rogue I still get the sneak attack, if the attack counts as a sneak attack. If you use it as a thrown weapon, it disperses at the end of the turn, but as a bonus action you can have it in your hands again on your next turn. In dark environments you get advantage on the attack which is good for the roguelock. 

((Note I usually use it with my offhand, and never throw it. That way my main hand can be a rapier improved hex weapon.)  

Oh, I get how the spell works. However, on D&D Beyond - there's no easy way to roll it.

So for example, the Rogue has dropped her dagger down a cliff side and has no short sword on their character sheet.

However, some enemies have jumped her - so she casts Shadow Blade and wants a sword.

Now, if she has no short sword on the character sheet - and the spell doesn't have it in there as "Roll Short Sword" and then it rolls the Short Sword Damage + 2D8 damage from the spell.

That's what I was trying to figure out an easy way of doing.

Just add a short sword to your character sheet with the note that its for the shadow blade roll only.

You can even edit it so it has no weight.