"(as) black as one's hat"の定義と意味

What is the origin of the idiom "black as one's hat" and when to use it?

The idiom "black as one's hat" is used to describe something that is very dark or black in color. The phrase originates from the fact that for about a hundred years, the vast majority of men's hats in Britain were black. This includes top hats, bowler hats, homburg hats, and others.