My source first heard this from a woman who was particularly interested in mysticism and was very superstitious. She apparently was a maid at his house when he was young, and one day she happened to see the bottom of his foot, on which is a small birthmark. When she saw it she exclaimed loudly, “Oh wow! You’ve been kissed by an angel!” According to her, a birthmark on the foot was the marking left after one had been blessed, though he thought it strange, he did not ask her why she believed so. My informant believed her nonetheless and continued to tell people the same thing if he encountered a person with a birthmark on his or her foot, suggesting that those people went on to tell others as well.
Where or when this particular sign is said to have come from, I do not know. However, the powerful nature of folklore when it comes to signs, especially if told to a person when young was very apparent through this collection. He did not question why or where the belief came from, but he believed and spread it nonetheless. It is likely that he will continue spreading this belief, and because I myself have such a birthmark, it is entertaining to believe it as well.