I've never seen a cougar in person but I have literally crossed paths with them.
When I was 9 or 10, I was camping with my dad in Capitol Reef National Park. I walked down the mooshie streambank for a mile or so, exploring. As I walked back to camp, I found mountain lion tracks in the mud ON TOP OF the footprints I had just made.
Chills.
My vote for your black cat is a melanistic jaguar or leopard that was some loon's pet and got released. They have a lot of evidence in Britain of breeding populations of big cats as a result of this.
I just don't see a jaguar making it that far north without help, and like has been said, melanistic cougars are undocumented.
Scary, either way, but cool!