The new Black setting in the adjustment brush is something I've wanted for a long time.
But it's seriously unstable. The more brushed black an image has the faster it crashes Lightroom. I have to resort to exporting to PSD and reimporting to resolve these crashes.
I've also found that this update has made switching from Develop to Library after a bunch of edits sometimes takes far longer--6-8 sec.--than previously. I suspect it may be mostly images that have just had moire applied via brush.
Try turning off the "Use Graphics Processor" in the Performance section of the Preferences.