Ttgjyuk

I've seen a number of sources cite Darkness/Devil's Sight as a combo for Warlocks in which the Warlock's attacks get advantage and attacks on them have disadvantage, due to the heavily obscured effect that generates the Blinded condition for those who can't see through magical darkness. This makes total sense to me for Pact of the Blade warlocks in melee.

What's not clear to me is whether ranged attacks from inside the Darkness -- such as Eldritch Blast -- still have advantage if their target is outside the sphere of effect. My intuition says that on the one hand, the target can't see the spell being cast, but the target could still see the spell coming after it left the bubble. I can't tell from the wording on heavily obscured areas or from blindness how this would work.

To be clear, the question is: assume a warlock with Devil's Sight is in the area of effect of a Darkness spell, and they cast Eldritch Blast at a target outside the Darkness area of effect. Do they have advantage on the attack roll?

They should still get advantage on the attack. DnD 5e rules say that advantage is based on whether the attacker can be seen, not the attack.

This is pointed in the "Unseen Attackers and Targets" section, on page 194 of the PHB:

When a creature can't see you, you have advantage on attack rolls against it.

The warlock has advantage

This is a notorious combo and it works because of the rules for Unseen Attackers and Targets (PHB 194):

Unseen Attackers and Targets

When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the GM typically just says that the attack missed, not whether you guessed the target’s location correctly.

When a creature can’t see you, you have advantage on attack rolls against it. If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

As the area of Darkness completely obscures the warlock from the other creature but not vice versa, the warlock is attacked with disadvantage but makes its attacks with advantage.