<< Supposedly the Zwartbles fleece is black, but the sun bleaches it to dark brown - much as it does Hebrideans, I think?>>
It's really just the tips which are bleached brown - deep in the fleece it's still black and after shearing you have a black sheep not a brown.
The situation with Hebs is apparently different. Many also have weathered wool tips, but in addition there is thought to be a brown gene, so there are fairly black Hebs (which are plain-chocolate brown when you look closely) and there are much browner Hebs. The brown type are frowned on by the showing fraternity, so most people pretend they don't have any. I have some
The tendency to bleach also seems to be genetic as some families do it and some don't.
With Hebs there is also a 'silver' type, which is a bit like the body markings of a katmoget Shetland ie black face, belly, tail and chest, but with a silvered 'rug' thrown over its back. Again frowned on by the showing fraternity but visible in the photo of the first known flock of Hebs, so an ancient characteristic - nothing to do with choosing a brown sheep