Oh I have info about the black sheep thing!
Have you heard of merino wool? It's a very reputed type of wool, the one that made Spain's wool industry famous up until the Industrial Revolution.
Merino sheep are originally black. Like these ones my parents have (recently sheared in those pictures):
They look brown-ish because the sun and the dust make it so that the outer part gets lighter coloured, the inner part or the wool in babies (lambs?) is actually of a quite dark and uniform black that turns more brown as they get older (again, the sun). If you notice, the skin in the head and legs is very dark black.
Those white sheep are also merino sheep but: that is the mutation.
Other than being very well adapted to the weather and space in Spain, they produce wool of good quality (about 2 kilos of wool per sheep), so the white sheep were chosen over the centuries because well, you can dye white wool easily. In recent years, some shepherds are trying to get back the original black merino sheep and they often work with artisans like these (where you can see what black wool looks like): https://dehesalana.com/
Now, I agree that the wool itself is more dark brown than black, but I guess with certain shearing (either shearing sooner than needed, or separating fibers?) or with certain dyes or chemicals, it can easily be dark black or dark colours.
When reading about local history (Extremadura, Spain) in the end of the 18th Century, many villages had "industry" (not a real factory per se, but many people making a living of this) of "paños pardos", that is "dark wool fabric" (paño is a type of work for the wool, where you spin it and weave it, and then felt it; pardo means dark brown). This is because, these sheep were frequent and the easiest way to dress.
Also, in Catholic countries, mourning for years was a very frequent thing (an aunt of my father spent over 10 years of her youth wearing mourning clothes, that is black, because different family members died), so obtaining black colours to dye all of your belongings was probably frequent and not that expensive.
But, in short, if the wool you have comes from black sheep, half of your clothing will be dark, if not black.
And to end, a nod to the Spanish speakers in the room. The idiom "no confundir churras con merinas" is in reference to two different type of sheep that are very common in Spain but not the same, it's used when something looks a certain way but don't be fool, it's actually a different thing.
