Aerosol sight black works well for me....spray it on before the match and will often go the entire match...if not....over to the safe area and re-spray. Have also used my carbide sight black lamp (Gunsmoke) with real carbide...left over from my NRA Service rifle days....gets things REALLY black and is non reflective as well....folks DO look at you however!
Bugler
I just put a quick swipe of black Magic Marker on the back of the front sight whenever I feel the need. Quick, easy, and cheep. Easily undone anytime, costs nearly nil.
Al