What does the idiom as black ace of spades mean?

The idiom "as black as the ace of spades" doesn't exist. The closest idiom you might be thinking of is "black as the ace of spades".

This idiom simply means very dark black, often used to describe something like:

* Someone's hair: "Her hair was as black as the ace of spades."

* A night: "It was a night as black as the ace of spades, you couldn't see your hand in front of your face."

* A cloud: "A massive storm cloud rolled in, as black as the ace of spades."

The ace of spades is traditionally black, and the comparison highlights the extreme darkness of the object being described.