cant use powers, black and white screen

hey guys ive just played through dishonored for the first time and there have been like 2 or 3 instances where i couldnt use my powers for a split second and the screen turns black and white even tho im full on mana. its not the dudes that block your powers with the music things. its not a big deal or that it was annoying, i just wanna know what caused it. ive tried looking everywhere but i cant find an answer. thanks in advance!