Hi, long-term player here, finally getting to play a warlock (and having an [Eldritch] Blast with it), and I have a clarification question. And a corollary with it as well.

Do you need vision on a target in order to target it with Eldritch Blast? It states "a target within range", not "a target you can see within range", so I would assume you don't have to - you would just have to know that the target is there. The question follows, of course, about invisibility and whether you can target an invisible creature with this.

And the corollary - does it have to be a straight line "beam", or can this spell curve around corners or a rock or the like? It is a beam of energy, and my DM and I aren't sure whether that requires a straight line, or if it can be bent to go after the target within range. Nothing directly in RAW states anything one way or another,  and I want to know how much I can Rule of Cool things now that I've wielding the dual Blast.

That's a fair enough explanation on the early part, and I don't mean to seem Devil's Advocate, but where is the statement that ranged attacks have no curve?Arrows make sense, yes, but this is a magical beam of energy (and doesn't state anything about "you must have a straight line of effect to the target", nor does it have the Line AOE type). I don't mean to ignore cover, it's more of a "Rule of Cool" style situation. Wide curves, while still respecting the point-of-origin rule for cover.

It came up in the session today thusly:
Fighting a group of grung in an underground cavern, some open spaces but one 5-ft or so stalagmite. Because of the placement of my other party members, I could not get a direct/straight line to the grung target, but could curve the Eldritch Blast around the rock to hit the grung that I did in fact know the exact placement of. My DM ruled to allow it in the moment, but decided to look up more information after the session to determine if it was allowable in the future, and I put this up to see if I could help with that. I'm not trying to use this to get around a creature having cover - no sharp turns or double-backs - but considering that I don't have to see the target in the moment to cast the spell, and this spell specifically targets creatures and not objects, it could be argued to have a bit of a "life-force seeking" bend to it, enough to curve wide around a rock. I was just wondering what other people thought on the situation, so thank you for your opinion!

No curving on ranged attacks means that you have to draw a straight line between you and the target.  You can say that your spell zigzags through the air as a thematic effect, but you can't claim that your Eldritch Blast is coming in from an angle to prevent your target from getting cover bonuses against it (unless you have some ability that explicitly allows you to ignore cover with a spell effect, like the Spell Sniper feat).

Quote from VanyelOrJustVan >>

I don't mean to seem Devil's Advocate, but where is the statement that ranged attacks have no curve?

[sic]

My DM ruled to allow it in the moment, but decided to look up more information after the session to determine if it was allowable in the future, and I put this up to see if I could help with that....

I’m not trying to use this to get around a creature having cover - no sharp turns or double-backs - but considering that I don't have to see the target in the moment to cast the spell, and this spell specifically targets creatures and not objects, it could be argued to have a bit of a "life-force seeking" bend to it, enough to curve wide around a rock.

 It is not explicitly stated that “all ranged attacks travel in a straight line” because it doesn’t have to be since ranged spell attacks and ranged weapon attacks all follow the same rules. There are not separate rules for each:

https://www.dndbeyond.com/sources/basic-rules/combat#MakinganAttack

Ranged Attacks

When you make a ranged attack, you fire a bow or a crossbow, hurl a handaxe, or otherwise send projectiles to strike a foe at a distance. A monster might shoot spines from its tail. Many spells also involve making a ranged attack.

Range

You can make ranged attacks only against targets within a specified range.

If a ranged attack, such as one made with a spell, has a single range, you can't attack a target beyond this range.

Some ranged attacks, such as those made with a longbow or a shortbow, have two ranges. The smaller number is the normal range, and the larger number is the long range. Your attack roll has disadvantage when your target is beyond normal range, and you can't attack a target beyond the long range.

Ranged Attacks in Close Combat

Aiming a ranged attack is more difficult when a foe is next to you. When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn't incapacitated.

https://www.dndbeyond.com/sources/basic-rules/combat#Cover

Half Cover

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

Three-Quarters Cover

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.

Total Cover

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

So no, spell attacks are not guided by your will as they traverse the distance from caster to target. They are fire-and-forget, just like an arrow.

That’s also why D&D has a rule for splitting movement and attacks on your turn. One common use is to move to where you have LoS, attack, and then move again to get back behind cover yourself.

As was stated, spells that force saves instead of requiring attacks can often get around that for you with one major exception, spells that specifically require Dexterity saving throws. Spells like these: https://www.dndbeyond.com/spells?filter-class=0&filter-search=&filter-save-required=2&filter-verbal=&filter-somatic=&filter-material=&filter-concentration=&filter-ritual=&filter-sub-class=

So the targets of any those spells also gain a bonus for being behind cover. That list includes Fireball by the way. 

There is however one unique spell in the entire game that is absolutely 100% guided by the caster directly to the target(s): Magic Missile. That’s the only one that completely bypasses everything.

I hope that was helpful.

Hold on, the Ranged Attack rules you cited only cover that a Ranged Attack Spell can only attack a target within its given stated range.
However, I think it's a mistake to equate a beam like Eldritch Blast to a fire-and-forget arrow because:

According to J. Crawford on a podcast dedicated to the topic of Targetting Spells:
- All spells must have an unobstructed path of travel to their target; total cover stops all spells unless expressly negated - an unobstructed path is not a line
- Only spells that state "a target you can see" have that requirement; he states a spell can travel through a thick fog or darkness otherwise just fine
- Some spells expressly state being a line aoe, this is the only thing forcing you to fire in a straight line

Eldritch Blast streaks to a creature within range (120 ft, no line aoe stated), has no requirement for "a creature you can see", and can therefore curve or turn to hit a target you are aware of within 120 ft. Finally, the assumption is that beams must always be straight lines - but that is not true (beams can refract, reflect, and bend) to say nothing of magic simply allowing them to.

Maybe it shouldn't work this way, and they should force the line aoe restriction on Eldritch Blast, but in 5e there's nothing RAW preventing you from convincing your DM this works. Of course, they could always just decide to rule that it must fire in an unobstructed straight line.

Quote from StriderStarfall >>

Maybe it shouldn't work this way, and they should force the line aoe restriction on Eldritch Blast, but in 5e there's nothing RAW preventing you from convincing your DM this works. Of course, they could always just decide to rule that it must fire in an unobstructed straight line.

 Yes there is, it's called Cover.

Trying to argue that you should be allowed to just blindly fire Eldritch Blasts at targets around corners is wrong and the sort of thing that your friends should throw dice at you for.