Both work. Same as carrying a lit torch into sphere of darkness. It stays lit while in the area of effect, and produces heat. When you exit, the light returns. 

Quote from Robermann_Mccloud >>

If the Druid is inside of a "Darkness" sphere (Spell) and cast the Flame Blade, is the Darkness dispelled? Or the Flame Blade negated? Or both work, but the Blade does not illuminate?

 The text of Darkness says "If any of this spell's area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled." So if you cast Flame Blade at 2nd level inside the Darkness AoE, Flame Blade will be dispelled immediately after being cast. But if you cast Flame Blade at 3rd level or higher, it will not be dispelled by Darkness.

Interestingly, you don't even have to be inside the area of darkness to have your light-producing spell dispelled, just close enough that the light produced overlaps with the area of darkness. So for Flame Blade that means anywhere within 20 feet of the edge of the darkness, or equivalently, within 35 feet of the centre (since the darkness has a 15-foot radius.)

Question. It says "you evoke a fiery blade in you free hand."

I assume by 'free' it means 'hand which does not hold the druidic focus (or leaf of sumac) you require to cast the material component of the spell.'

Now, if that's correct, not a huge deal, you have two hands. Except that any druid silly enough to go into melee combat will want a shield in the other hand... (druids aren't clerics, they can't have their shield as a focus).

So. If that's true, it would seem that you cannot use this spell and hold a shield.

I could be wrong, its late.

Honestly It might be better to reflavor shadow blade with fire damage and emiting light than using this one.

I can't find this spell on my druid? She is level 2 i just cannot find it

the way i read darkness it doesn't destroy the spell, just the light created by it

talk to DM for sure, but something like a single leaf wouldn't be a problem at my table, especially if you had warcaster

Quote from bleddyn05 >>

the way i read darkness it doesn't destroy the spell, just the light created by it

 As always, run your table as you like, but I think RAW for darkness doesn't leave much room for interpretation when it says "the spell that created the light is dispelled".

I think this is a good thing BTW, since it gives darkness a bit more utility. All too often it's just a fancy way of blinding friend and foe alike.

Level 2 druids don't have access to level 2 spells.

Yeah i found out

Quote from jdahveed >>

Why is this only available for druids?

 And bards, with magical secrets. But yeah, why is that?

Probably one of the worst spells in 5e. 3d6 damage a turn is not a lot, and when you're level 3 or 4 a 2nd level spell slots are too valuable to be used on low damage per turn. But once you reach level 5, this spell manages to become even worse. It averages 10.5 damage, compared to Primal Savagery's 11, and being a cantrip Primal Savagery doesn't need a spell slot. This spell can be upcast, but if you have 4th level spell slots, 4d6 damage per turn is really low.

If the caster is Hasted, can they use the extra action given to strike twice with this? The thing that makes me wonder is that Flame Blade says you use an action and Haste grants you an action that at most grants one attack.

Sorcerer can use it as well, I think

Quote from vulcanrock >>

Do you think that it can be used in wildshape? I was thinking a giant ape with a flaming blade would be quite a sight to behold.

 You can't become a giant ape without the polymorph spell.  

Quote from Osicat >>

How do this spell work with circle of spores? Do I still get the 1d6 poison dmg or is it better do Shillelagh + say booming blade

Quote from Phlynch >>

I would assume you would, since you're attacking with a weapon. It's a conjured weapon, but it's still a weapon.

 Symbiotic Entity specifies that the extra poison damage only applies to "weapon attacks."  Flame Blade isn't a weapon attack, it's a spell attack, so thus it wouldn't work with Symbiotic Entity.  Maybe your DM might rule otherwise, but RAW:  no.  

The description of the spell explicitly notes the attack made with Flame Blade as a "melee spell attack," not a "melee weapon attack."  

This is a bad spell.. 1 million times worse than shadow blade which is same level

According to a Dec ‘21 clarification, it can! Which has interesting implications for spell attacks going forward. Now with M3’s fire genasi, you can have a Charisma Paladin with an upscaling fire sword without any sort of Hexadin shenanigans; and you can have a Flame Blade Bladesinger (if you pick scimitar as your proficiency) that can be used with the Blade cantrips, benefits from Extra Attack, and scales, but also benefits from anything that improves your spell attack bonus.

Shadow Blade and Flame are what they should have been—accuracy vs. power. A Flame Blade will be more accurate natively, while a Shadow Blade can be more devastating in darkness.

Quote from Fahrai >>

According to a Dec ‘21 clarification, it can! Which has interesting implications for spell attacks going forward. Now with M3’s fire genasi, you can have a Charisma Paladin with an upscaling fire sword without any sort of Hexadin shenanigans; and you can have a Flame Blade Bladesinger (if you pick scimitar as your proficiency) that can be used with the Blade cantrips, benefits from Extra Attack, and scales, but also benefits from anything that improves your spell attack bonus.

Shadow Blade and Flame are what they should have been—accuracy vs. power. A Flame Blade will be more accurate natively, while a Shadow Blade can be more devastating in darkness.

 

What part of the errata are you referring to? RAW, I still don't think you can't use the booming/green flame blade cantrips with Flame Blade or Shadow Blade as they require a material component of a melee weapon (specified as the one used in the spell's casting, so no off-hand shenanigans) worth at least 1 silver, which the Flame Blade is not.

Quote from Magumbasauce >>

nope. I haven't looked at all of the "smites", but the ones i did see "melee weapon attack" while this spell says "melee spell attack", which is pretty much the same reason (i assume) for booming blade/green flame blade. Though in my game, I might allow smite for those (BB and GFB) because they are using the paladin's normal physical attributes for the attack. And since this is specifically a "Druid" spell, even though it's a weapons, that would be a hard sell in my game to allow. But per RAW, no. 

Besides those reasons–assuming a DM was inclined to overlook them–the smite spells are all concentration, which would require dropping the Flame Blade spell concentration.