a blot on the escutcheon

A stain or mark against one's reputation or that of one's family. An escutcheon was a heraldic shield that bore a family's coat of arms, and thus serves as a metaphor for one's honor. I was at the top of my class in college, but that narcotics charge will be a blot on the escutcheon for years to come.

bringing dishonor or shame to oneself and one's family. “Escutcheon” is the heraldic term for the shield on which a family's coat of arms is painted. One with a blot, in the sense of blemish, would look as though there were something wrong, which is what The phrase is meant to convey. When Uncle Fred was hauled off to jail for embezzlement, he not only disgraced himself, he brought shame to his wife, his children, and any living parents. His crime was thus a metaphorical blot on the family escutcheon.