It’s kind of interesting, how many times black canids show up associated with death, and specifically the transition/boundary of death.
Off the top of my head, you’ve got Anubis who embalmed the dead and guided souls to the afterlife. You’ve also got Cerberus, guarding the gates of the land of the dead. But my favourite image in this regard–where my brain goes quickest, and what makes me smile–is Black Shuck. Black Shuck is an East Anglian creature of folklore; he’s a harbinger of death, and seeing him means you’re probably going to die soon.
Surprisingly enough, despite the less-than-friendly demeanour depicted in the image above, the stories mostly don’t seem to mean “soon” as in “as soon as he chews your leg off”. More of a “as soon as you have time for the full horror to sink in and to tell a few people” thing.
(That picture, by the way, is one I ran across around age six. I think I promptly adopted Black Shuck as the most awesome imaginary friend ever. You can do that kind of thing, when you’re six.)
To tangent briefly: the term Black Dog to refer to depression–as in the Black Dog Institute, or the “I had a black dog” animated video–was drawn from Churchill’s references to his bad moods as “a black dog on my back” (something akin to “getting up on the wrong side of the bed”), a colloquialism that also refers back to the black dogs of folklore.
I suspect black dogs have mostly been on my mind, however, because I recently reread Bob Leman’s “Loob“–in which the Goster County dogs are not friendly, not approachable, but ultimately a signifier that things may return to an idealised order, and are described as follows:
…almost a distinct breed, huge rangy dogs with blunt muzzles and smooth black pelts, who stood baleful guard over the farms of the county and patrolled the streets of the town with a forbidding, proprietary air.
It kinda fits.
 As to Norse mythology: I’m honestly not sure if Fenrir fits this mould or not. Canid, yes; associated with death, yes if you count the end of the world as a death; black, I really could not tell you. Garmr is a blood-stained watchdog that guards the gates of Hel, but again: could be lilac for all I know.
 Probably isn’t lilac.