Melanistic jaguars seem to have a wider variance than melanistic leopards based on my zoo observations (and I have seen quite a few cats). At my own zoo (Reid Park - Tucson) we have two female littermates, yet one is quite dark with the spots almost invisible and the other is lighter with the spots easily visible like the cat pictured here. Not quite as light as the one in this photo appears to be, but that may just be because the photo is slightly overexposed. Since camera meters adjust to a middle tone, dark subjects tend to get overexposed (note how washed out the foreground cement is), so the cat in this photo may not be quite this light in real life. Black leopards, on the other hand, seem to be universally dark with the spots only faintly visible in bright light.
I don't think it's been that heavily exaggerated by the camera - I've also come across a large spectrum of black jaguar variations, including this similar one in Peru: http://www.zoochat.com/1108/black-jaguars-180452/
