## It is not the Blinded condition that gives advantage/disadvantage ##

The advantage/disadvantage comes from **Unseen Attackers and Targets** (PHB p.194):

>When you attack a target that you can’t see, you have
disadvantage on the attack roll.

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

The creature in the darkness can't see the creature outside the darkness and vice-versa. Therefore they each have advantage and disadvantage which cancels out.