Kuro-rekishi (黒歴史) is often cited in anime and other sub-culture media and resembles “dark history” – or a period during your youth where you may have acted/produced content that when you look back in several years makes you want to put your face in the pillow and scream.

Typical references of Kuro-rekishi include situations such as pretending to have a different personality within or having special powers. The term for acting these ways are also referred to as Chuunibyou (中二病).

Terminology – Chuunibyou (中二病) - Japan Anime Education

Remember when you imagined that you had some hidden superpower to summon dragons from the hells, or you are actually a secret descendent of a forgotten god? We've all been there - just around when you are in the 8th grade - or a disease (byouki) that you get in Chuuni (2nd grade in middle […]

https://japanime.tv/terminology-chuunibyou

Although reference in anime for Kuro-rekishi is often cited towards these moments, it can have a broader reference, generally relating to past events that you simply want to forget whether it be that awkward boyfriend from 10 years ago, to having dyed your hair in a weird color.