Dash action is compromised both by the inside being difficult terrain and by blindness.
The location the caster centered the darkness matters. If I set it with 30′ in front and 10′ in back, an enemy that is blinded and unable to tell boundaries will face a dash of 30’+30′ through difficult terrain the long way, leaving them just inside the area.
At that the repelling blast invocation combined with eldritch blast is nasty.
The eldritch blast splits into 2 beams; when it hits, each beam knocks back the target 10 feet (blast is not reduced for difficult terrain).
The target takes the initial 2d6 damage, runs to the edge and ends for another 2d6. On the next turn, they get repelling blasted back into the area of effect for another 2d10 (if both hit). On the start of their next turn take another 2d6. Still being inside the dark area, they can’t see so direction matters. This sets up the chance to run 30′ (out 20 and back 20 – just inside, because as they exit terrain changes to normal for movement.)
If they dash out and you blast them again, you’re pushing them back 20′ into the darkness dealing another 2d10, then they take 2d6 more starting the next round. If they moved normally and could now attack, as long as they are not pushed back by the front line before the warlock blasts them again, they would finally be clear.
Fighters stopping movement just at the wall exit for the warlock to blast them back is very effective.
In the above combination, it offers 2 rounds of damage, up to 4d10 + 8d6, for being inside the dark area. If they use dash there is no attack on you or your friends.
And just because you can’t see and light does not travel doesn’t mean a fireball or other area effect spell won’t work as well. Combo hunger of Hadar to halt them, then fireball the area.