* "Swearing black and blue": This means to swear vehemently or emphatically, often in anger or frustration. The idea is that someone is so insistent on the truth of something that they would swear it even if it were a color that doesn't exist, like black and blue.
* "Swearing up and down": This idiom also means to swear vehemently and insistently. It describes someone going through all the motions of swearing to prove their point.
Let me know if you meant one of these idioms!