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.

What matters is who can see whom.

If the darkness is normal darkness (i.e. an absence of light), the creature in the darkness can see the creature in the light. The creature outside the darkness would need some special ability (like darkvision) to see the creature in the darkness.

If the darkness is magical (e.g. the Darkness spell) you cannot see through it as well as not being able to see into it, barring a special ability (like Warlock's Devil's Sight), neither can see the other.

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.

What matters is who can see whom.

If the darkness is normal darkness (i.e. an absence of light), the creature in the darkness can see the creature in the light. The creature outside the darkness would need some special ability (like darkvision) to see the creature in the darkness.

If the darkness is magical (e.g. the Darkness spell) you cannot see through it as well as not being able to see into it, barring a special ability (like Warlock's Devil's Sight), neither can see the other.

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.

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.

What matters is who can see whom.

If the darkness is normal darkness (i.e. an absence of light), the creature in the darkness can see the creature in the light. The creature outside the darkness would need some special ability (like darkvision) to see the creature in the darkness.

If the darkness is magical (e.g. the Darkness spell) you cannot see through it as well as not being able to see into it, barring a special ability (like Warlock's Devil's Sight), neither can see the other.