The Curse/Mark of Cain
What we know of the mark, and curse, of Cain comes from Genesis Chapter 4, verses 10 to 15, in which God confronts Cain about the murder of his brother Abel. When considering your question, then, it is worth quoting from the passage at length:
‘11And now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand. 12 When you work the ground, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth." 13Cain said to the LORD, "My punishment is greater than I can bear. 14Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me." 15Then the LORD said to him, "Not so! If anyone kills Cain, vengeance shall be taken on him, sevenfold." And the LORD, put a mark on Cain, lest any who found him should attack him’. (ESV)
The first thing we can take from this passage is that there is a distinction between the curse that God placed upon Cain as a means of punishment, and the mark which He then placed as a means of protection. We shall now consider each in turn:
The Curse of Cain:
Cain’s punishment is linked to his crime. Cain will no longer be able to cultivate the soil (v11-12) because his brother’s blood has cries out to God from the ground upon which he was slain (v10). Ironically, God chose to focus upon depriving Cain of his primary skill, which was to work as a labourer of the ground (Genesis 4:3). God also stated that Cain would be a fugitive, and a wanderer upon the earth.
In your question you asked whether the curse of Cain could have been that of black skin. Categorically, the answer to this question must be no. The curse God placed upon Cain was described in the passage; namely, an inability to cultivate land, which uniquely fitted both Cain’s vocation and his crime. There is no indication given anywhere in the passage that the curse also applied to Cain’s descendents and, quite apart from this fact, the unique nature of the curse itself, and the plain reading of the text, mitigates against the idea that it applied to his descendents.
Logically, the effects of this part of the curse could not possibly have applied to black people, who, historically, have been unaffected (like all other people) in their ability to cultivate land.
The Mark of Cain:
Verse 15 states that God put a mark upon Cain so that he would not be attacked by anyone who happened to venture upon him. Despite much scholarly speculation, the precise nature of this mark is uncertain. The word translated as "mark" is 'owth’, which is used around 80 times in the Hebrew Scriptures, to signify or describe events ranging from a sign in the stars, to a miracle performed by Moses.
As such, the text only describes how the mark was to function as some sort of sign (the most common translation of the word) and not what form the mark took. Whatever it was, it must have been something visible to have served as an indicator to others that Cain was not to be killed. Some propose that the mark was brand of infamy, perhaps a disfiguring scar, or tattoo, that would have served as visible mark of God’s judgement. The Jamieson-Fausset-Brown commentary speculates that it may have been some wild ferocity of aspect that “rendered him an object of universal horror and avoidance”. Beyond this, however, we know nothing, and as John Wesley remarked, any further “conjectures of men are vain”.
The Curse of Black Skin?
Finally, I want to more fully consider the teaching that the mark of Cain is tied to the curse, and that the curse is one of having dark skin. This heretical idea gained popularity during the 18th century in Europe and America, and is also written in the false book of Mormon (which originated from around that time).
Although this racist teaching has now been eschewed by the overwhelming majority of Christian Churches, it was popular during the 1700s - early 1900s as a means of legitimising both discrimination against black people, and institutional slavery in Africa.
However, this teaching is blatantly sub-biblical. The first, and most obvious, reason for this is that the curse of Cain is an inability to cultivate land, and has nothing to do with skin colour. Secondly, as was mentioned above, the text strongly favours the interpretation that the curse applied solely to Cain, and was not passed on to his descendents.
Yet another reason for dismissing this doctrine is the fact that Cain’s descendents most likely died out during the global flood. Noah’s family was the only one to survive the flood, so even if God had placed upon Cain come sort of inheritable genetic trait his descendents would have intermarried with Noah’s family, distorting the unique characteristics of any such trait that could possibly have been passed down.
Further, the many black people mentioned in the Bible, including Job, Hagar and Moses’ wife Tzipporah, were at no point identified as being partakers of the curse. In fact, the third High Priest of Israel Phineas, the grandson of Aaron, was given a Nubian (a black Kushitic people) name. It makes little sense for such an esteemed figure in Jewish society to be given a name common among the accursed, particularly at a time when the book of Genesis was itself being compiled.
There are many other persuasive reasons for dismissing this doctrine, including the fact these interpretations did not exist until European colonization, that the Bible (in Jeremiah 13:23) draws a distinction between marks on the skin and skin colour, and finally that such interpretations were unheard of in the Early Church, within which many of the leaders and prominent figures were black.
Even if every one of the arguments touched upon here were to be soundly refuted, the pendulum still swings back to the original purpose of the mark, which was to protect Cain from being harmed. It seems axiomatic to point out that the mark is, in fact, completely unconnected to the curse. The logical concomitant, then, is that all a mark of dark skin would signify, were it to exist, is that Africans are descended from Cain, leaving men still grasping for some basis to justify their sin. Sadly however, man has, and will continue to, attempt to twist Scripture to fit their own preconceptions, rather than allowing the word of God to break them down.
Justification for ill-treatment or discrimination against others can be found nowhere in the Bible; and the very idea stands in stark contrast with one of the most foundational teachings of the New Testament, found in Galatians 3:28, that all men are equal in Christ: “There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus”.