Bit out of my usual patch this one - I'm based up in Northumberland where we get our own big cat reports fairly regularly, mostly around the moors. But the pattern you're describing sounds very familiar. Large, black, low-slung, moving along treelines at dusk. That description comes up again and again across completely different continents and I don't think its coincidence.
What I'd want to know is the rough size estimate relative to something fixed in the landscape, and whether anyone caught the gait. Big cats move very differently to dogs or deer, that slow almost liquid shoulder roll is quite distinctive once you know what you're looking at.
There have been serious researchers who think there's a genuine relict population of melanistic pumas in remote parts of North America, officially extinct from the east but possibly not. Worth documenting these sightings properly rather than just reporting them informally. Photographs, timestamps, GPS coordinates if you can manage it. The cumulative record does matter over time.