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.