I have nearly 100 hours playing the game -PC version- and something happened that turned it dark on me as well. It's so bad that in daylight building features are so dark, almost black, I can't tell what they are when I am standing across the street. At night it's way worse. When fighting off waves of bad guys from any distance I can't distinguish who the bad guys are and who my guys are. Driving is very difficult. This just happened recently after I bought some DLC content via Steam..
No other games or applications have darkened except Saints Row. Nothing in the game settings has any effect. I have a pair of geForce GTX 580 Fermi Black Ops video cards and using the nVIDIA control panel has zero effect to make it playable without trashing the way everything else looks. What's odd is that when the game loads for just a second it looks OK then the over darkening effect kicks in.