a blemish: a physical defect or moral fault that mars someone or something otherwise whole.
At its heart this word names a flaw that spoils something otherwise whole. Most often it is a bodily defect: a broken bone, a blind eye, a deformity, anything that mars a body.
The word does its heaviest work in Israel's worship. An animal brought to the altar had to be perfect, and a priest who served there had to be sound in body; a single defect ruled either one out. So "without blemish" became a kind of stamp of fitness for holy use.
Away from the altar the same word simply describes bodily perfection. Absalom was famously handsome with no flaw from head to foot, the beloved in the Song is called flawless, and Daniel's young men were chosen partly for being without defect.
From there the word stretches naturally to the inner life. A clear conscience is a pair of hands with no stain on them; a corrupt generation is marked by its fault. English does the same thing when it speaks of a blot on someone's character, which is why the Greek μῶμος that later renders this word can mean both a bodily defect and a disgrace.