I believe (it has been a while since my last physics class) that a bright enough source of non-visible light would distort your vision creating the sensation of seeing black, without it actually being black. Basically, it would be causing you to not be able to see visible light through it, the it wouldn't appear invisible, but as black (your eyes lacking the senses to translate them into anything it can understand.)
"Ghost Legions? Bunch of common criminals... they shouldn't give us any problems sir." - Last transmission of Col. Henry Driver, 4th Tau Ceti Lancers.