Genesis 4:9–12, "Then the LORD said to Cain, “Where is Abel your brother?” And he said, “I do not know. Am I my brother’s keeper?” 10 He said, “What have you done? The voice of your brother’s blood is crying to Me from the ground. 11 “Now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand. 12 “When you cultivate the ground, it will no longer yield its strength to you; you will be a vagrant and a wanderer on the earth.”"
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.
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.
Is the curse of Cain that of black skin? No, it isn't. 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.
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.”
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.