God marked Cain to protect him from vengeance for Cain killing Able. (Gen. 4:15) Therefore the mark wasn't skin-motivated -- nor was it in response to having committed murder...but was actually a sign of mercy that protected Cain...
God later established cities of refuge for manslaughterers to go live to protect them from vengeance by victims' relatives.
Jesus Christ then became the Ultimate Refuge who would save even manslaughterers and murderers from their sin.
Only some 19th century (and for Mormon, 20th century as well) people linked the "mark of Cain" as being skin-linked and a curse.
Gen. 4:11-12 shows the curse upon Cain was to be exiled...a vagabond...and unable to grow his own crops...not any mark put on him.
Cain's descendents came down thru Noah...so who is claiming that Noah was black?
Gernesis 4:15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.