Premise

A character with the Alert feat (PHB, p. 165) has been blinded and is being attacked.

One of the benefits of Alert is:

Other creatures don’t gain advantage on attack rolls against you
as a result of being unseen by you

One of the effects of the blinded condition is:

Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage.

Question

Are attack rolls against the character made with advantage?

No

This is an instance of specific beats general. The feat grants you immunity to granting advantage when you can't see the target; it doesn't have any additional criteria for that.

For the purposes of abstracting the matter, consider that Perception isn't just a measure of visually seeing something. It also includes hearing someone moving or using other senses to detect the presence of something. From the PHB:

Perception Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses...

So a creature that's unusually alert enough to the point that they've the Alertness feat is someone whom isn't just seeing you. They're hearing you before they see you. Or smelling you before they see you. Or feeling vibrations in the wind before they see you. Or whatever else, to the point that you don't get Advantage to attack them unseen.

No, being unseen does not create an issue for someone with Alert

While the Blinded condition does not reference Unseen Attacker rules, that is in effect what's going on when you're blinded. You are blinded, you can not see, therefore the attacker gets advantage on their attack.

The Alert Feat says that unseen attackers do not get advantage for being unseen. You are blinded, but not being able to see the attacker does not affect you so the attacker does not get advantage.

But what does Alert really mean?

The wording is...not the best. And Jeremy Crawford has stated such but not actually created an errata for it:

The 3rd benefit of the Alert feat is imprecisely worded. It's meant to work against creatures you can't see.

By RAW:

Yes, because the Blinded condition is not based off Unseen Attacker

When you are blinded, it is more than just that you cannot see people. You cannot see the ground, you cannot see your sword, you cannot see the table your swing will hit.

If the Blinded condition simply stated "everyone is unseen by you, so you suffer disadvantage on attack rolls and they gain advantage when attacking you", then the Alertness feat would nullify it.

It does not read that way. So the effects of blindness are not dependent on being able to see your attackers/target or not.

Rules do what they say. Blindness makes you suffer advantage/disadvantage.

However,

it is reasonable for a DM to decide that Blinded only has impact due to everyone being Unseen, as the rules are basically identical.

So the first paragraph of Blinded occurs (fail all Perception checks involving sight), but not part of the second (granting advantage).

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:

1. They hear small sounds of the approaching attack


2. They see movement in their peripheral vision, and react instinctively (or, if the attacker in invisible, they notice disturbances in the surroundings)


3. 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.

Always on the lookout for danger, you gain the following benefits:......
lookout - the act of looking out
outlook
looking, looking at, look - the act of directing the eyes toward something and perceiving it visually; "he went out to have a look"; "his look was fixed on her eyes"; "he gave it a good looking at"; "his camera does his looking for him"

When attacking a blinded creature that has alert, the attacker has advantage.
Go for the eyes Boo....