A mark on something that spoils its appearance. (Cambridge Dictionary)
A small mark or flaw which spoils the appearance of something. (Oxford Dictionaries)
A moral defect or fault. (Oxford Dictionaries)
A blemish is a small mark on something that spoils its appearance. (Collins Dictionary)
A blemish on something is a small fault in it. (Collins Dictionary)
If something blemishes someone’s character or reputation, it spoils it or makes it seem less good than it was in the past. (Collins Dictionary)
To spoil by a flaw. (Merriam – Webster)
Your Dictionary. Blemish.
How Many Different Kinds of Facial Blemishes Are There?