How to say embarrassing dark past in Korean
Korean: 흑역사[ heuk-yeok-sa ]

Meaning
흑역사 is a slang word for an embarrassing thing from your past that you do not want to remember. People often use it for old photos, bad fashion, silly mistakes, or awkward videos. It is common in casual conversation and is often said in a funny way.
Example sentences
그때 찍은 사진은 흑역사야.
That photo from back then is an embarrassing dark past.
친구가 나한테 말했어, 그 옷 입은 건 정말 흑역사였어.
"Wearing that outfit was really an embarrassing dark past for me," my friend said.
그때 찰나의 실수는 내 흑역사야.
That momentary mistake is my embarrassing dark past.
친구가 나한테 말했어, 그 춤은 정말 흑역사였어.
"Dancing like that was really an embarrassing dark past for me," my friend said.