Like the others have posted, it’s a pretty common recommendation. I believe part of why it works is that the white dots or bars on some rear sights tend to attract your attention and focus. Blacking out the sight makes it easier to acquire and focus only on the front sight. I have a few handguns that I’ve painted the front sight bright yellow or orange for more contrast between the sight and target. A base coat of white paint before you paint the bright color seems make the color pop a little more. I find this particularly beneficial on my older revolvers with very minimal sights. Also, if you practice shooting with a handheld light you may find that night sights aren’t absolutely necessary. Index the light from different positions on your body and see how it affects your sight picture, you might find a particular index that suites you better.