Black Stone was White

Ibn Abbas narrated that the Messenger of Allah (sal Allahu alaihi wa sallam) said: "The Black Stone descended from Paradise, and it was more white than milk, then it was blackened by the sins of the children of Adam." [Jami at-Tirmidhi - hasan hadith]

This narration is a proof that when the Black Stone descended from Paradise it was white and became black by absorbing the sins of the children of Adam (alaihis salam) who touch it.