As Rockin noted; ones chance of seeing a normal spotted jaguar, which rarely show up along the Mexican border with Arizona, in fact only two or three in the last 30 odd years. The chances diminish exponentially as you move north, away from the Mexican border.
The melanistic phase, only occurs in about 6% of an all ready small population which live in a specific area of central America. The odds of seeing one of those, even smack on the border with Mexico, would be astromonical, considering the very small chance of seeing a normal spotted variety.
Imagine the realistic odds of seeing a black one in Alabama, where everybody has a third cousins brother who swears his grand daddy saw one, but his rifle misfired, or he didn't have a camera or any other witness......the story line is generally the same in all the sightings.
Ive been fortunate to see my fair share of Mt. Lions, while either calling predators or just out hunting and tramping around the desert, twice seeing pairs, with the rest singles. Depending on the time of day, a lions coat can "appear" to be very dark, when in late evening and low light. Put him in the shadows while light is getting dim, and it might just look like it's black...I suspect this is the real reason for the black lion sightings?