I'm going to disagree slightly with some of the folks here - at my table, it would depend on the situation.
From a conceptual standpoint, what is it that an alert person does to prevent being easily attacked by an unseen attacker? I would say it's a combination of:
- They hear small sounds of the approaching attack
- They see movement in their peripheral vision, and react instinctively (or, if the attacker in invisible, they notice disturbances in the surroundings)
- They're aware of their surroundings and anticipating where an ambush might come from
By my reckoning, not noticing the dude standing behind the door is quite a bit different from not even being aware of the door's existence. So, for me, it would come down to the question "did this player have a reasonable chance at predicting where an attack would come from?"
Let's say someone throws a handful of dirt in your eyes to blind you. You were just looking at your surroundings, and you know where they were, so you can still have a pretty good idea where the attack is going to come from, so they wouldn't get advantage.
What if you're blinded for a while, and not in territory that you know the layout of? Let's say you've been blinded by some magical source, and you're wandering around in a tunnel, bumping into the walls. You have very little clue what's around you, and you probably don't even realize an enemy is present. In this situation, all you have to go on is sound (unless you have something like Blindsight), and I would grant them advantage.
Last but not least, my understanding is that if it's not explicitly stated, it's not a rule. As blindness does not explicitly state that the advantage is due to them being unseen, I interpret that as evidence that there's more to being blinded than just not seeing your enemies - not being able to see your surroundings can significantly impair your ability to move in combat, especially if you're accustomed to sight.