[Bizhvywt]

“...ok, what is the mark of Cain?...the general thought of society until a few decades ago was that black skin was the mark of Cain.”

Weren’t Cain’s descendant lost in the Flood? Aren’t we all descended from Noah?

[JAKraig]

One of Noah’s sons, Ham saw him naked so Noah cursed his son, “Cannan”. Cannan was to be a servant of servants. Tradition says the curse was black skin. Many people think this was the excuse the Israelites used to subjugate black Africans.

[Elsie]

Weren’t Cain’s descendant lost in the Flood? Aren’t we all descended from Noah?

Genesis 6:10
Noah had three sons: Shem, Ham and Japheth.

We are not told WHO Noah's wife or the son's wives were descended from.

Or the wives of all of these guys; all the way back to ADAM:

Seth, Enosh, Kenan, Mahalalel, Jared, Enoch, Methuselah, Lamech.

[Elsie]

...the general thought of society until a few decades ago was that black skin was the mark of Cain.”

It mattereth not what 'society' thinks; but what the BOOK says:

GENESIS 4:9-16

9 Then the LORD said to Cain, “Where is your brother Abel?”

“I don’t know,” he replied. “Am I my brother’s keeper?”

10 The LORD said, “What have you done? Listen! Your brother’s blood cries out to me from the ground. 11 Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand. 12 When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.”

13 Cain said to the LORD, “My punishment is more than I can bear. 14 Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.”

15 But the LORD said to him, “Not so; anyone who kills Cain will suffer vengeance seven times over.” Then the LORD put a mark on Cain so that no one who found him would kill him. 16 So Cain went out from the LORD’s presence and lived in the land of Nod, east of Eden.