Keep in mind that dogs too are associated with witchcraft but in Europe at one point it seems to be more frequent in Britain which is also where you have dogs being that associated with witchcraft, almost on par with cats based on what I’ve read. This also goes hand in hand with reports of black dog ghosts though sometimes those dogs weren’t dark themselves.
It seemed the demonisation of dogs in Britain was so strong that it could’ve shaped contemporary black dog lore to the detriment of actual black dogs even if it would’ve happened before. Black dog lore is also apparent in other countries but nowhere as marked as in Britain. Suddenly, Shakespeare’s disdain for dogs not only makes sense but was more commonplace than thought.
There was even an article called ‘The Great Dog Massacre’ and that in Elizabethan England*, far more dogs were killed than cats and pigs are. That could be an isolated incident but when in light of black dog lore, perhaps witches’ dogs would’ve helped if not inspire such tales.
*Demon Possession in Elizabethan England.