I have the same issue, nothing seems to be really broken, rather it seems there isn't just any function implemented in the program for pencil erasers until now.
Of course I can use the E-button, but some pencils (mine is a Wacom Grip pen) are made with the usual pencil tip on the one side and an eraser, a more thick tip, on the top of the pencil. The clue is to just have to turn around the pencil to use the eraser and not have to press a special button or something like that.
In other programmes it does work, I think Black Ink can't differ between the two different pencil tips. In the Windows settings for pencil input I can also chose the point 'use end of the pencil to erase (if available)', so I think it's just not available in Black Ink right now.
Am I right or did I miss something important?