A recent update for Radeon HD 7800 (and possibly other models) caused almost all BLUE colors to display as BLACK on the screen in Photoshop CS6 (I'm running Win 7 64 and CS6 64) if your image is in an RGB Profile.
If you drag the image window around, the blue shows normally. If you take an eyedrop sample of the color, it is actually blue... but it still looks black in the image window. If you change the image profile to CMYK, the blue looks fine. Change it back to RGB and it's black again.
Adobe support people on chat, with a shared screen, messed with my system and settings, uninstalled and reinstalled, deleted all of my preferences and profiles... basically goofed around for over 4 hours and couldn't figure out any solutions.
After I got them off my computer and spent almost two hours getting all of my settings and profile set back up, I did a trace for changes on my system. I found that AMD/ATI had done an update, Catalyst 13.1, the day before, so I rolled back my video driver and... PRESTO. Problem solved.
The stable dirvers are 12.8 and 13.2 beta FYI.
Thank you for your post. ATI has known about this problem for quite a while, so they shouldn't have been goofing around.
Confirming what Curt has said, I've been running Catalyst 13.2 beta 5 for quite a while and it's very stable - and solves the aforementioned problems.