I shoot with both eyes open, so it helped me a great deal. I just used a sharpie.
It didn't make me a better shot, as much as it helped sight alignment and confidence.
The way I see it, is the fundamentals are what makes you a good shot. Choosing a sight picture that best suits you (blacked out rears for me), is a bit of fine tuning that better rounds out the shooter's technique that leads up to the moment, right before the trigger is pulled.
I look at shooting as a 3 point event (A, B, and C). A being at the ready, B being the draw and pressing out, and C being taking the shot. I'd put blacking out the sights in point B, if that get's my point across any better.