Excited to report that I may have an answer to both instances of "black jaguars" mentioned here

This specimen was
photographed at the Zoo Hannover in June, 1967. In the
original Facebook post, its pelage can be compared to that of a regular, tawny jaguar and a fully melanistic individual from the same zoo.
Another
photo from the same thread shows the same cat, though possibly at a younger age?, alongside a regular jaguar. My German is rusty, but does the caption mention the pair being born at the London Zoo? Or vice-versa?
The above scan is taken from Dittrich, L., 1979. -Die Vererbung des Melanismus beim Jaguar, (Panthera onca). D. Zool.Gart. NF. Bd. 49, H. 6., which describes the cat as "partially melanistic".
I would bet
@Newt's photo in question is of the same individual, though it would be interesting to know where said photo originated. Perhaps the same publication? Or an old newspaper article? Anyways, hope this helps !