When I select an image in LR CC (2015.5.1) the image goes dark, and then after a second or so, it appears as it's proper brightness level. I have been using Lightroom Since the first public beta and never saw this behavior. Nor do I see this with Photoshop or Capture One. Is it a bug in this version, or something else? In the end, the exported files are fine.
This could be due to graphics card processor.
Would suggest you to go to Lightroom Preferences > Performance > uncheck "use graphics processor" > relaunch Lightroom and then, check.