Sounds like a video issue.. but I usually don't display graphics of the work units.. then again not a good idea since my GPU is crunching projects when not in use too... Nvidia GTX 580 3 GB
I have now checked the M0000__ type tasks i had and the graphics are looking normal and working, so i don't believe the black graphics for the C0000__ tasks, to be a card or a driver problem.
I think there is something is wrong with them, although they are still getting validated & credited.
ps/ I don't look at or run the graphics all the time either, never have.