The human race has a number of species. Almost all of them evolved as per Darwins theory from the ape over a period of many thousands of years. Most of this development of the races because of the evolution from different species of apes also evolved differently.Thus we have the Chinese and Japanese races which are different from the white race and blacks.
The Black race generally is associated with Africa and because of the harsh climate and lack of development, led to this race having to struggle more to survive.This again is Darwins theory of the survival of the fittest. The black or the negroid race thus devolped greater physical characteristics than other races and in popular ideas a feeling grew that they are also more potent in the sexual field.
A review of hard core porn will show that the greatest demand is for a coupling of a black man with white or other woman. This fantasy is played to the ultimate hilt, though there is no scientific evidence to back this claim. What panders to this fantasy is the physical characteristics in terms of height and build of the black man which tower over other races. These physical attributes do not add to greater potency, but then in popular imagination a man or a woman would like to believe this is true.
This myth commenced from the plantations in the deep south of the USA, where the African was made to work day and night on the farm. The result of this hard labor , was a greater develoment of the muscular aspect of the body and the house ladies sitting in smug comfort of their mansions were attracted to these muscular slaves. Obviously in contrast to the sedate existence of the white master, the Black scored in physical built.
Another aspect that feeds this fantasy is the human minds desire to try the bizzare in life.This is also one of the reasons that the myth of the black man as a more potent lover has gained credance over more rational thought. But whatever the real fact, this myth of the black man being a more potent lover will not easily go away.