If you don't want to do anything permanent to your gun there are a couple of classic ways to do this.
This aerosol product, Birchwood Casey Sight Black, claims to do the trick:
http://www.amazon.com/Birchwood-Case...ds=sight+black
and then there is the really classic technique of smoking your sights with a carbide device. It's discussed at lenght here:
http://www.ar15.com/archive/topic.html?b=1&f=5&t=543769