Why and when did the misconception about the Panther being a species came to be?
Tigers, lions, jaguars and leopards are all different species. Panthera is a genus, a broader classification than a species. Tigers are Panthera tigris, lions are Panthera leo, and jaguars are Panthera pardus. In each cases the formal species name is the genus followed by the species identifier. Common usage rarely gives a hoot about the subtleties of biological classification. 

Your initial impression was correct: any member of the Panthera genus that has a black coat is called a black panther. In South America they are jaguars, in Africa and Asia they are leopards.