If you haven't yet sacrificed that quest, Schipperke, why not try loading up the game and using options to change to 'windowed mode' before opening up the problem Knight save file?
I had a similar darkness problem just now. My screen went black and the game minimised. It did this because another program had interrupted my game. When I maximised the game, everything in live mode was a dark sepia, as if someone had turned out the lights. Very difficult to see what was going on. I saved, quit and reloaded and, fortunately for me, everything was back to normal when I re-started.
However, I've come across this problem before. It happens to me with The Sims 3 too. If I should ever minimise the game then, when I go back to it, it is too dark to see what is going on. (It's impossible to press C to get a picture to show this, as the pictures come out at the normal brightness.) If I want to browse the internet whilst playing TS3 then I have to play in windowed mode. I don't like windowed-mode though, so I don't. I also try to turn off any background programs before playing.
Hopefully, changing to windowed mode, just to complete this quest with the knight, will be a short-term answer to your problem, Schipperke.