Randomly my tool bars will blacken. In Develop and Grid. So far I haven't found a recurring reason and the only way to get them back is by re-starting the program.
I have used Lightroom for 8 years and now and this hasn't happened before.
It's definitely not associated with toggling (shift+tab) or darkening to a lightbox (L key).
After the last update it didn't do it for a longer stretch, perhaps it has something to do with memory/cache?
I did recently add more memory to my iMac 5k, but so far no other problems...
There is a known bug on Mac versions of LR6.8/2015.8 where the panels all turn black intermittently. See Link:
I don’t know whether this is related but another user has also reported the same problem. You could follow that thread for a forthcoming update or start a new bug report.
Adobe has been working closely with Apple but I don’t think the testing was completed in time for version 6.9/2015.9. If you prefer, and have no immediate need for new features such as Reference View you could roll back to the previous version. But that means uninstalling, re-installing LR6.0/2015.0 and then downloading the LR6.7/2015.7 patch.