I'm working on house rules for advantage/disadvantage that makes the number of each matter, instead of any amount of each cancelling out. For this project I need to know how blindness and unseen targets generate disadvantage.

I don't know whether to consider the blindness condition and unseen attacking as two separate rules or one. Blindness gives (among other things) disadvantage to the creature's attacks against others.

My interpretation is that this is because they can't see attackers or targets.

An example situation is that a blinded Ranger shoots an arrow at another archer based on his last known position. He is blinded and the target is unseen. Is this two disadvantages or one?

This is part of why the "adv and dis cancel out, no matter how many" rule exists, so you don't have to ask questions like this. ^_^ (In other words, it doesn't matter whether they're the same or not, the player still just gets a single Disadvantage, which can be cancelled by a single Advantage.)

However, stepping outside of RAW, then yes, interpreting blindness as just invoking the "unseen attacks" rule is reasonable. Being blind doesn't inherently make you clumsier, you just can't see.

The Blinded condition and the act of attacking an unseen target are 2 independent sources of disadvantage.

These are separate rules that happen to be very similar and govern many of the same situations, but they are still separate.

So if you care about counting sources of disadvantage, a creature having the Blinded condition has disadvantage on its attacks, a creature attacking a target it can't see has disadvantage on its attacks, so a creature that has the condition and does the action has disadvantage from 2 different sources.

It's completely up to you, as the author of your house rule.

In the official rules, your question has no meaning - there is no distinction between "one source of disadvantage" and "two sources of disadvantage". You just either do have disadvantage or you don't have disadvantage, the disadvantage doesn't have a magnitude.

Because you are developing a house rule which creates a distinction that does not officially exist, you have to define the parameters of that distinction for yourself. We can offer our opinions about where we think those lines should be drawn, but there is no objective answer to your question.

They are two separate sources of disadvantage.

Player's Handbook, p. 290, under the Blinded condition:

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

Player's Handbook, p. 194, Unseen Attackers and Targets:

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see.

D&D 5th edition's philosophy is that a rule means what it says, and no more. Here there are two things which both grant disadvantage, and no rule which says that those are the same thing. Thus, there are two sources of disadvantage.

The DM can, of course, always rule otherwise, if in their opinion such a ruling makes sense.

Note that the standard rule is that multiple sources of disadvantage don't matter, for two design reasons: counting multiple sources slows gameplay, and a character stacking multiple advantage for an extremely high chance of success would be unbalanced (D&D 3rd edition could devolve into a game of stacking lots of small +1 and +2 bonuses this way).

A creature which is blind will always also qualify as being unable to see its target, since, you know, it's blind. This means that if you consider these as two separate sources of disadvantage, then a blind creature will pretty much always have two sources of disadvantage right off the bat.

That might be reasonable; for example a sighted creature attacking an invisible target might not be able to see their target, but they will be aware of their surroundings and footing, whereas a blind creature might trip over a rock or something, making them even more likely to fail. In the other hand, having the automatic extra disadvantage this might make blindness such a crippling condition that it ends up being less fun because it's so difficult to succeed.

Ultimately, you have to decide which interpretation makes sense in your homebrew rules but you should make sure that you focus on keeping things fun for the players.