After you've fixed the colour space, try these solutions: http://www.damiensymonds.net/2015/09/black-boxes-and-other-weird-behaviour.html
I'm not optimistic, though, to be honest.
That's ok, I was actually going to blame the calibrator, but if you don't have one, it can't be the culprit
What about if you open a jpeg file into Photoshop? (No ACR involvement). Does it look ok? Is this problem only happening with raw files?
No, it was quite specific to this photo. Another photo of the same scalp might require different numbers, and a different scalp might require an entirely different method altogether.
Great. So, click on the mask of one of those layers, and apply a little bit of Filter>Blur>Gaussian Blur, to make the edge just soft enough to be plausible. Then repeat on the mask of the other layer.