Still happening 
I am having same issue, What I have done to resolve the issue is when the small box opens, move mouse and click on top of the blizzard game box before the same opens, then click on your mactool bar, this should release your mouse. If your too late you can still try and move the mouse to top edge of screen and click and try and move mouse, Works for me everytime. I hope they fix this issue.
Oreyo
command M did work for me, was a bit easier
thanks
Same issue here. Using Mac. Com + Tab goes to black screen. Com + M works, but doesn’t allow me to utilize any other applications on my computer that I might need to jump too. Disappointed with a game I love so dearly (and pay for monthly). I might not be repaying if the issues persist.
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Hi! What solved the black screen issue for me was disabling the “automatically switch graphics card” (not sure what it is called in english as mine is in portuguese) in the energy settings. A bit late but hopefully still useful!
This helped too here. Thank you so much 