One question that arises from the account of Cain and Abel is what exactly is the mark that God placed on Cain? Speculation ranges from the Hebrew letter "tav" (ת) all the way to some disfigurement that would terrify his approaching brethren. Sadly, among the many theories is the idea that Cain's mark was black skin, not only designed to set him apart from his brethren, but also passed down to his descendants. This interpretation is, thankfully, very rare in our modern day, but it still exists, particularly within certain heretical groups which shall remain nameless, but was also somewhat prominent among racists of the not-so-distant past, such as American slave traders.
Of course, this is justified nowhere in the actual text. Let's read it: "And the Lord said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground. And now art thou cursed from the earth, which hath opened her mouth to receive thy brother's blood from thy hand; When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth. And Cain said unto the Lord, My punishment is greater than I can bear. Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. 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. And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden." (Genesis 4:9-16).
What, in any of that, even hints at the mark being black skin? We aren't told what the mark is, full stop. It's even possible, although it is less likely, that God never marked Cain at all, but rather, He may have given Cain a token or sign. The word for "mark" here is ‘owth (אות), which, in verses like Judges 6:17, is translated as "sign". Nevertheless, the dominant view is that God did, indeed, place a physical mark on Cain. The chances of it being black skin, however, are roughly... 0%.
For starters, consider the folly of such a theory. In order to give rise to all the nations (Acts 17:26), Adam and Eve would, themselves, need to be medium brown. Therefore, Cain and his siblings, as their immediate descendants, would also be similar shades of medium brown. The response to Cain's skin being darkened, therefore, wouldn't be especially dramatic. It would be more like "wow Cain, you've been spending a bit too much time in the sun...".
Aside from the fact a change in skin tone wouldn't be an especially noticeable mark to Cain's family, there is also no indication that his mark would be heritable. People get all kinds of marks from all kinds of places. I have stretch marks and tattoos, will I pass those to my kids? My tattoos are all black, after all. There's no indication what so ever that the mark is passed down.
All of that tells us that those who believe/d that the mark of Cain is black skin, and that that's where black people came from, are illiterate. They either can't read, or can't comprehend what they're reading, no matter how simple it is. But let's ignore that. Let's grant their silly assumptions. Why did God give Cain the mark? According to the passage, the mark is about the most merciful thing God did here. Yes, He punished Cain, but the punishment is that he will be a vagabond (basically a wanderer), and he would forever fail at farming.
The mark is not the punishment. Rather, it is a defence against further vengeance! God set the mark on Cain "...lest any finding him should kill him." So if you're one of those numpties who believes black skin is the mark of Cain, what you're really saying is that black skin is a sign of God's personal decree "if anyone kills this man, I will avenge him sevenfold". Well doesn't that just put a damper on racism against black people?
While we can never know what the mark of Cain really was, we can be absolutely sure that black skin wasn't it. Anyone who claims that black people bear the mark of Cain forfeits all credibility, as it is not only an intellectually bankrupt theory, but also a morally reprehensible one. Racism against any race is very unChristian. Using the Bible to justify it in any way is like eating a spoonful of salt to quench your thirst.