Hello, Elle!
No, you would not know that phrase.
I have hunted and discovered that it is a Scots phrase.
Originally, "As black as the Earl o' Hell's waistcoat".
The Earl o' Hell was the Devil.
The saying meant pitch dark.
In certain parts of the country Earl Haig was substituted for the Earl o' Hell.
I rue mentioning him now!
I completed the crossword, in spite of the Cockney clue.
A bit wet here, and we may get snow.
i have been gathering old ladders, and iron boards, etc .to take to the skips in the morning.