Black dogs?

Well, the UK there's a rich history of black dogs/demon dogs in folklore.

These supernatural entities could sometimes protect a lone traveller at night on a deserted country lane, all the while keeping pace with their chosen individual.

Alternatively, and more commonly, they do (as you suggest) spell doom or misfortune to any who encounter them and can be seen as malign spirits or even as an aspect of Satan. Again, a lone traveller will encounter them, usually at night, and the dog will attack or behave in a sinister way. Soon after misfortune or death will fall.

Regionally, these creatures go by many names but I'd recommend that you search for "Back Shuck" online for more info.

So, in summary, you're either being protected or you're going to have a bad time soon. Or, they're just some silly black Labradors!

Since you used "vacation" I assume you live in the US so you probably needn't worry: I don't think demon dogs can swim!

First, dogs don't "wonder" the streets. Please look up the difference between wonder and wander, then you'll know the difference.

Black is merely a color of dogs, cats and other animals and it only means that the animal has a lot of dark pigment and nothing more. To be so superstitious as to think that seeing a black dog means you're going to die soon is ridiculous. You don't need to scare yourself this way.

I've seen a black dog every day of my life for the last 16 years, sometimes more than one black dog per day. I'm still here.

Black dogs are pretty popular.

There's one sitting next to me on my sofa right now.

No.