Not from the Pacific Northwest myself (Lancashire, UK) but this is actually more common than people think and probably not as mysterious as it sounds. Black squirrels are just a melanistic colour variant of the grey squirrel, same species basically. We get them over here in parts of England too, Letchworth and a few other spots have loads of them. They look almost unnatural when you first spot one though, I get why it'd make you do a double take.
That said if you're seeing something genuinely large or behaving strangely that's a different conversation. What made it stand out to you beyond the colour? Was it bigger than a normal squirrel or acting odd in any way? Genuinely curious because sometimes these sightings turn out to be something worth digging into and sometimes its just a melanistic animal doing squirrel things.
