I'd like to know what is the earliest reference for Black Dogs such as Black Shuck, Hairy Jack, Barghest, etc. Is there anything before the 1577 Black Shuck of Blythburgh in worldwide folklore? And are there any recent "sightings" or articles referring to it? I need to precise that I don't mean in ancient mythology as in Cerberus or Geri and Freki, but more like in legends and folklore worldwide.
I'm not sure about black dogs off-hand, but Welsh folklore has the Cŵn Annwn or "fairy hounds" which are white dogs with red ears and eyes, and have very similar legends other than that, of being omens of death or taking the dead to the afterlife. Annwn, the "otherworld", is the home of fairies and the dead in Celtic mythology.
The Cŵn Annwn appear in Pwyll Lord of Dyfed, one of the stories in the Mabinogion - the earliest known copy of this is from the 1300s, but it's thought to be much older even than that. That story doesn't mention any connection with the dead, though, apart from the mere fact that it mentions Annwn (although it spells it "Annwvyn" for some reason). Their only role in the story is that Pwyll encounters a pack of them while out hunting, and steals a stag which they've killed. Their master turns out to be Arawn, the king of part of Annwvyn, who sets Pwyll a task to make amends for stealing the stag. I don't know whether the association with death was already around then, or, if not, when it started.