noun
dark past; cringeworthy history; something one wants to forget
Internet slang for embarrassing or shameful past events, often from one's youth, that one wishes to erase or pretend never happened. Commonly used self-deprecatingly.
中学時代のポエムは完全に黒歴史だ。
The poems I wrote in middle school are totally a dark past I want to forget.
昔のブログを見つけて黒歴史を掘
I found my old blog and ended up digging up my cringeworthy history.
歴史 is the standard word for 'history'. 黒歴史 is a slang compound meaning 'dark history'.
Compound of 黒 (kuro, 'black', metaphorically 'dark, shameful') and 歴史 (rekishi, 'history'). Originated as internet slang, likely from otaku culture, referring to embarrassing past creative works or behavior one wants to hide.