This is the northern most documented sighting as of 2004.
The photo was shot where the red star is. It's not in the U.S., but the border is only about 250mi away, well within the jaguar's range of travel for food/mating, so I suppose a sighting here is possible.
According to the article, melanistic pumas (mountain lions)
have been reported.
"Melanism in jaguars has now been shown to be caused by a dominant mutation of a single gene (Pocock 1929, Kitchener 1991 and Sunquist and Sunquist 2002). Thus, melanism has been reported from a total of 13 of 36 species and 9 of 18 genera of the family Felidae (Wozencraft 1993) including (besides the jaguar): the leopard (Panthera pardus), lion (P. leo), tiger (P. tigris), caracal (Caracal caracal), tiger cat (Leopardus tigrinus),
puma (Puma concolor), bobcat (Lynx rufus), margay (Leopardus wiedii), ocelot (L. pardalis), serval (Leptailurus serval), sand cat (Felis margarita), and kodkod (Oncifelis guigna)."
Full article (not to long, and very interesting) here:
http://dinets.travel.ru/blackjaguar.htm