How to practice tactics correctly? When it comes to tactics training what is the best approach to optimize results? I've recently started only doing really hard puzzles, the only immediate change is it obviously take me a lot longer and end up maybe doing around 10-20 per day. Is it quality or quantity that's more important? Also as a side question I keep hearing that playing blindfold chess is bad for you for some reason? Can someone explain why I keep hearing that?
I'm assuming your aim in practicing tactics is to improve your game performance. 
In that case, I would recommend you treat each problem not as a puzzle, but as a real game situation, because your goal is to be able to spot tactics OTB, without knowing whether or not a tactical shot exists or not. 

Thus, you should apply the thinking process you would use in a real game to the puzzle, and just try to play the move you would in a real game.

This means that you would, after a quick evaluation of the initial position, look for (1) Forcing moves, checks/caps/threats, (2) logical, positionally sound options that increase your activity/decrease opponent's activity. Then, you should calculate and evaluate these candidate moves one by one, and determine which one is the best. This is how you would play in a real game, and thus your tactical training should be like this.

If you can't spot the tactic (get the problem wrong), then you know that something is wrong with your thinking process, and you can correct it immediately. For example, if you spotted all the appropriate candidate moves, but, after calculation/evaluation, didn't find a winner, and thus settled for a "positional" move, then you know that there's an issue in your calculation; maybe you didn't remember to find all candidate moves at every move, and thus missed something. Or perhaps you couldn't visualize a long sequence, and now you know you have to get better at this. 

This is assuming you know how to improve every aspect of your thinking process. There are many free online resources, particularly youtube, that tell you how to train to improve certain things.

I think that 10-20 difficult puzzles is plenty, as long as you apply your thinking process to each, and remember to diagnose/improve your thinking process at the end of each puzzle. Also, remember that in a real game, you have around 3 minutes per move. In critical situations, you may use more, but try not to use more than x minutes for a move, if you not spend more than x minutes on the position in a game. 

Also, playing blindfold chess is not bad. Blindfold chess is one way to improve your visualization skills, which is crucial for calculation. 

I am a USCF expert rated player.