GRiD made black PC's before the NeXT. https://en.wikipedia.org/wiki/Grid_Systems_Corporation

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".

There was also the black apple ][ by Bell and Howell

Oric Atmos and Telestrat, also black: https://en.wikipedia.org/wiki/Oric

Commodore Plus/4, C=16, C=116.

The NeXT cube was a workstation, not 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:

https://en.wikipedia.org/wiki/SGI_Indigo

I wish we saw more of that bluish metal like in the SGI Indyigos or the classic netgear hubs & switches.

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.

I got one of those after my time at SCO, supporting Z-mail. It was quite an upgrade from Wyse 60s running at 9600 baud.

There was a black version of the Apple II for the education market (1979)

http://oldcomputers.net/bellandhowell.html