How do you improve your blindfold chess?

 As with other chess related skills (like endgames or calculation or whatever) the answer is really boring:

practice the skill you want to improve.

You want to improve blindfold play? Then play blindfold chess. Can't complete a game? Doesn't matter. Go as far as you can. Record the moves. Visualize each position as clearly as you can, then play over the recorded moves on the board to compare reality with what you visualized.

Do this a few hours a day, for a few years, and you'll be great

"but that sounds really boring"

Yeah, it is boring. That's why most people can't play blindfold chess. But if you love chess for its own sake, and not for titles or parlor tricks (like blindfold chess) then you'll do this sort of practice on your own, for fun, because you're sick in the head