Since the release of Lightroom 6.8 and CC 2015.8, some users have reported that their panels in the Library module suddenly go black:
This seems to only be an issue with Mac OS Sierra. Update: also El Capitan. The quickest way to get your panels back is to switch from Library (G) to Develop (D) and back or hide and then show the Lightroom application. If this doesn’t work, close and reopen Lightroom. If the bug is intolerable for you, consider rolling back to Lightroom 6.7 / CC 2015.7 until it’s fixed. UPDATE 3/6/17: I have been advised that Adobe has seen reports of this issue in 6.7 / 2015.7 and earlier versions as well. Nonetheless, reports of it have jumped dramatically with 6.8 / 2015.8. Therefore there’s no guarantee that reverting to 6.7 / 2015.7 will solve the issue for you, but there is a good chance. Note that this issue has not been fixed in 6.9 / 2015.9.