Simply put, Accountants record their numbers in different colored ink. Positive balances in black ink (good), and negative balances in red ink (bad).
It depends on how you use it. If you mean literal colors, then it's not an idiom. If you say something like "It's all there in black and white," then it's an idiom meaning that something is printed.
It means to be very angry.
There aren't any idioms that mean "black" that I know of. There are plenty of similes, like "black as the ace of spades."
This is not an idiom. It is comparing one thing to another, so it is a simile. Remember: "AS ___ AS___" means A Simile! It is just saying that something is very black.
In debt. Its antithesis, "in the black," refers to having a balanced budget.
"A black day" is an idiom in contemporary English that is relatively uncommon but still quite useful. With "black" referring primarily to darkness (of night, of a storm, etc.) when there ought to be light, its basic meaning is simply this: "a bad or difficult day."
It's not really an idiom. It means "what are you thinking about."
It is not an idiom, it means your nose is itching.
RFP is not an idiom. It's an abbreviation.
A black sheep is one which does not do what all the other "sheep" (people) do - a person who does not follow cultural laws and morals, but does whatever they want.
"Sieve" is not an idiom. See the related link.
It's not an idiom. It means the tip of your nostril.