Stain, Smudge
noun
A dirty point is a little point of dirtiness on something. This is a stain or smudge.
おてん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
人生の汚点は日本語では黒歴史とも呼ばれる。
A disgrace in life is also called dark history in Japanese.