The expert on this subject is Mark Norman who operates 'The Folklore Podcast'. He produced an episode on the Black Dog folklore and he published a book by the same title.

Norman is strong on documentation, but doesn't offer a great deal by way of explanation. He suggests that the tradition may be related to the Wild Hunt - a widespread European tradition. Ghastly dogs - figuratively and sometimes literally 'Hell Hounds' - are known in several cultures including the Celts and the ancient Greeks. It's more famous recent literary manifestation is Holme's case of the Hound of the Baskervilles, celebrating a ghostly dog associated with an English household of fiction.

I don't have much more than this to offer, but you will certainly find the podcast of interest; Norman does a great job with this productions.

I hope this helps.