When under the blinded condition, all things are treated as having total concealment from you:

All opponents are considered to have total concealment (50% miss chance) against the blinded character.

When a creature has total concealment, attacks cannot be made against them (though you can attack the square you think they're in, which is what allows the 50% miss chance rule) and AoO's may not be made against them:

You can't attack an opponent that has total concealment, though you can attack into a square that you think he occupies.

You can't execute an attack of opportunity against an opponent with total concealment, even if you know what square or squares the opponent occupies.

As such, blinded creatures can be considered to not threaten any squares, and as such cannot provide flanking.

Assuming, of course, that they do not have any other method of determining precise location (blindsight, tremorsense, etc...) and are not adapted to being blind (through extended time being blinded, usually years)

No because grappling does not involve an attack roll

The Blinded condition is described as:

Attack rolls against the creature have advantage, and the creature’s Attack rolls have disadvantage. (basic rules)

As you rightly say, grappling in the PHB calls grappling a “special Melee attack” which allows you to make an ability check to apply the grappled condition to a creature.

Using at least one free hand, you try to seize the target by making a grapple check instead of an attack roll... (PHB p. 195)

It is a contest, not an attack roll. No attack roll means no advantage. Simple as that.

So, for your final question, none of those are correct, but #2 is close. Because Grappling is technically an attack, it is a special melee attack action but it requires no attack roll.