What is 3. A dark vein or spot on a lighter material, as in marble; hence, a blemish or defect; as, a cloud upon one's reputation; a cloud on a title called?

cloud — 3. A dark vein or spot on a lighter material, as in marble; hence, a blemish or defect; as, a cloud upon one's reputation; a cloud on a title. (Webster's 1913.)

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