The name was given (possibly) by Henry Stanley.
The reason was that very little was known about Africa - other that its external borders until the Scramble for Africa. Livingstone and other explorers in the middle to late 19th Century "discovered" and mapped internal Africa.
I speak under correction but another reason may have been that internal Africa was assumed to be peopled by "savages" as in living without the light of "civilisation"

No b lie. Evrytin abt Africa is dark. Dark minds, dark thinking dark evrytin. Just check 4rm d type of leaders we av. Y i no come white self..