Hey Santaman2000,
Leopards are smaller than jaguars, and it is melanistic leopards that have been referred to as black panthers. (yeah yeah yeah, all from the genus panthera, the large felines that have different hyoid bones that allow them to roar...etc etc....) Since leopards and pumas are pretty close to a match on size it may be technically correct that pumas aren't "big cats", but from a conversational standpoint, being that correct would dictate that we describe pumas as large small cats
.
In the UK, when talking about non-native-larger-than-tabby-puss cats, they all get bundled under the banner of "big cats", even if some of them don't roar, and might be no bigger than a bobcat.
For black cats, my money would be on black leopards, and I would not want to put money on there not being, now or in the past, some roaming around the British countryside. Black jaguars are fairly rare, and black pumas are so rare as to be practically mythical (despite various eye witness accounts), but black leopards turn up in zoos and private collections fairly often. Leopards live from the South African Cape up to Siberia, in every environment including big cities, even more adaptable than pumas, but tend to be a bit less secretive. If folk didn't keep saying the cats were black, I would be backing the idea of pumas too.
Chris