A blemish is a small flaw. If you borrow your brother's brand new bicycle, you'd better be careful — what might seem like the tiniest blemish to you might be a horribly ruined paint job to him.
The classic example of a blemish is a pimple or other mark on your skin, but you can call any small defect a blemish — like a worm hole in an apple or a scrape on the side of your car. A flaw or fault in your personality or morals can also be called a blemish. You might say that the one time your dog accidentally nipped you is the only blemish on his reputation as a sweet, gentle pet.