Thick Fog, Black Veil, Covered Up
noun
A black fog is going to be a really thick fog or black veil that covers suspicious acts or crimes. It's not a literal cloud of fog or a physical veil, it's what you hide your bad deeds in, metaphorically speaking.
くろいきり
The reading comes from 黒い and 霧.
ホームステイ先のお父さんが、今日の夕飯の際に黒い霧事件について説明をしてくれました。
My home stay father explained the Black Mist Scandal to me at dinner tonight.