So I was blind and the creature I was attacking was blind too. So attacks against them have advantage but I attack with disadvantage, I'm assuming these cancel out and its just a normal roll. First, is that correct?

Then if I attack recklessly do I then regain advantage or is all just null?

One instance of disadvantage cancels all instances of advantage and vice versa. As soon as you have any number of advantages (>0) combined with any number of disadvantages(>0), you are rolling straight rolls.

Advantage and disadvantage cancel each other out, yes. Additionally, it does not matter how many of either you have: if you have three sources of advantage and one source of disadvantage, they still all cancel, and you make the roll with one die as normal.

What they said. It may seem weird, but it keeps it simple.

Quote from _Amunsol_ >>

So I was blind and the creature I was attacking was blind too. So attacks against them have advantage but I attack with disadvantage, I'm assuming these cancel out and its just a normal roll. First, is that correct?

Then if I attack recklessly do I then regain advantage or is all just null?

 Ordinarily, it's all just null because Advantage and Disadvantage stack to normal, as Saga said.  The only extra bit to tell you about is that rules that remove disadvantage stack normally to remove disadvantage.  So, let's take your example: a blind person recklessly attacks a blind person.

• Your question:  Unseen Attacker = Advantage, Reckless = Advantage, Unseen Target = Disadvantage:  Target is a normal roll to be hit.  Likewise, when the target swings back, normal roll to hit, as there's at least 1 Advantage and at least 1 Disadvantage.
• Now, suppose you don't attack Recklessly and do have the Alert feat.  Your roll to hit is still normal, but when the attacker attacks back, they have a source of disadvantage (being unable to see you) and have no sources of advantage (you didn't attack Recklessly and have the Alert feat, which removes advantage for them being unseen).
• Similarly, if you remove disadvantage to Stealth checks caused by armor - e.g. using the Medium Armor Mastery feat or Mithral armor - and have Advantage to Stealth checks, e.g. via the Enhance Ability spell, you'll have advantage on the check.

So, recapping:

• If you have at least 1 source of Advantage and at least 1 source of Disadvantage, the roll is normal.
• You can independently find ways to remove sources of both, and those stack.
• If all sources of Advantage are gone and you have at least 1 source of Disadvantage, the roll is at Disadvantage, and vice versa.

And what about multiples disadvantages? I know it is specified but.... It doesnt seems right i mean:

1 archer use a Short bow to shoots an enemy at 210 feets, diadvabtage couse distance. 

Anorher archer use a short bow to shoot a enemy at 210 feet, after being cursed for omen, in magical darkness, with the enemy prone and with 1 arm down. 

Same deficulty.... 

Same in case of advantages, i think they should stack. 

Maybe you have a +2 for each stack ofadvantages over the initial advantage,  and -2 for each stack of disadvantages over the 1st stack. That sound a bot more reasonable to me. 

Same thing, multiple disadvantage don't stack. But the DM can always decide to apply penalties or bonuses to your attack roll when Making An Attack

1. Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

2. Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.

3. Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

Quote from Dakri >>

And what about multiples disadvantages? I know it is specified but.... It doesnt seems right i mean:

1 archer use a Short bow to shoots an enemy at 210 feets, diadvabtage couse distance. 

Anorher archer use a short bow to shoot a enemy at 210 feet, after being cursed for omen, in magical darkness, with the enemy prone and with 1 arm down. 

Same deficulty.... 

Same in case of advantages, i think they should stack. 

Maybe you have a +2 for each stack ofadvantages over the initial advantage,  and -2 for each stack of disadvantages over the 1st stack. That sound a bot more reasonable to me. 

 It’s the way it is for simplicity, not because it makes sense. You can rule it however you like in your game. Personality I like it as is just so it doesn’t bog down combat any more than it already is while players calculate how many advantages vs disadvantages there are

I’d say just toss half or three-quarters cover into that second case if you really think it needs it. Might be playing a little fast and loose with cover rules there, but it’s one way to reflect some additional obstacles without getting too punitive about it.