Not just you! I've seen one twice now near my property out here in western Washington. Solid black, not just a dark brown one like you sometimes get - pure black like a shadow. First time I thought I was imagining things honestly.
Did some digging and apparently melanistic squirrels do exist and can pop up in eastern grey squirrel populations, but they're rare enough out here that I still did a double take both times. Some folks in my area think they're a bad omen which always catches my attention given my interest in the stranger side of things lol.
Would love to know where roughly you spotted yours. I'm curious if there's a particular area they keep turning up or if its more scattered sightings across the region.