Black jaguars and spotted jaguars are the same species. The dark coat of black jaguars occurs because of melanism—a greater-than-average deposition of black pigment. Some melanistic jaguars have visible spots while others are solid black.
Many people apply the blanket term "black panther" to any large, black feline. Black panthers are not a separate species but rather an existing species that exhibits the genetic mutation "melanism." Melanism is simply the overabundance of dark pigment and appears in some tigers and leopards, as well as jaguars. In most cats melanism is a recessive trait, meaning the cat must possess two copies of the mutant allele to exhibit black coloration. In jaguars, however, melanism is incompletely dominant. A jaguar with a single copy of the mutant allele shows darker than average coloration but is not as dark as in individual with two copies. Despite being a dominant trait, melanism occurs in only 6 percent of the jaguar population. There is speculation that melanism is more locally common in regions with denser forest where dark fur provides better camouflage.
Some black jaguars are actually pseudo-melanistic; "abundism" is another name for this mutation. Rather than an increase in overall black pigment, abundism causes markings to be overabundant. In some cases, the spots are so abundant that they run together, causing the pseudo-melanistic cat to appear totally black.