Was the NeXT cube the first black PC? Here's a good look, for those of you who have never seen one: http://www.johnmiranda.com/next.htm
There was also the Amstrad CPC: https://en.wikipedia.org/wiki/Amstrad_CPC
And the Sinclair ZX81: https://en.m.wikipedia.org/wiki/ZX81
And the Memotech MTX: https://en.m.wikipedia.org/wiki/Memotech_MTX
PDP-11's were mostly black, but I suppose that doesn't count as a "PC".
The distinction is important, because workstations were office equipment and never consumer equipment. They had much fatter margins and a bigger industrial design budget.
For instance, here's another workstation from the same era as the NeXT cube:
We had some pizza-box style Indys in a computer lab when I was at uni. They were some of the prettiest computers I'd ever seen, even though they were just rectangles.