”Don't I also recall a lot of Border Morris teams stopped blacking up and used different colours, despite not being anything to do with race and skin colour?”

One of the reasons they stopped blacking up was precisely because it was felt by many, although of course not everyone, to be racist and/or racial stereotyping.

And as someone who worked for thirteen years for a company which made coloured plastic in primary form (i.e. granules and powder form, raw materials for manufacture of plastic objects), I can tell you that black is amongst the cheapest, if not the cheapest, colour to manufacture. And, of course, black plastic bags are highly opaque.

I’m pretty sure the choice of black for refuse sacks is a case of pragmatism and economics, nothing to do with racism or trying to offend people of colour.