Chris, Iíve encountered exactly the same phenomenon on my iMac some months ago. It happened while in the develop module. Hitting the ďGĒ key and returning to the library module only rectified the situation momentarily. Every time I returned to develop. The side bars blacked out. Frustrating!
I closed Lightroom CC, and then started up again and all was fine for a day or two. I thought it had something to do with RAM (my iMac has 8GB). I kept turning LR off, then on again for a few weeks. In the past four weeks or so, the problem hasnít returned. Maybe some update rectified a bug. I canít recall having more software running in the background than in former times. It remains a mystery to me but hasnít returned, I almost forgot Ė until I read your post.