Blemish, defect, flaw all denote an imperfection.
Blemish applies to something (as a spot or stain) that is external or superficial and mars or disfigures the appearance of an object.
Defect implies the lack or want of something which is essential to completeness or perfection but which is not necessarily superficially apparent.
A flaw is a defect in continuity or cohesion (as a break, a crack, or a fissure), that may either mar a perfect surface or cause a weakness in structure.