I gave a lot of thought to this question some years back and I came to the following conclusion:-

If PC is to work, it must work for everybody.

So as far as colours are concerned (in the UK), you should avoid not only black, but all of these as well:

Red – Upsets the workers
Orange – upsets liberals
Green – tree huggers
Blue – Tories
Pink – Gays
Yellow – Cowards (though they are usually too frightened to complain)

Only purple seems immune, being linked to nothing more contentious than fastest lap indication at GPs.

What did I get from all this?

The PC Song

Scarlet, Amber, Cerise and Jade, Tangerine, Purple, Azure,
I Think that it's a rainbow, hope it's a rainbow, but I'm not quite sure.

Don T.