When Sam was searching for Frodo and had reached the highest tier of the tower of Cirith Ungol, he heard two orcs arguing. During the orcs' argument, the orc captain, Shagrat, alluded to the "Black Pits" in the follow quotation:
'Then you must go. I must stay here anyway. But I'm hurt. The Black Pits take that filthy rebel Gorbag!' Shagrat's voice trailed off into a string of foul names and curses. 'I gave him better than I got, but he knifed me, the dung, before I throttled him. You must go, or I'll eat you. News must get through to Lugbúrz, or we'll both be for the Black Pits. Yes, you too. You won't escape by skulking here.'
The Return of the King, Book VI, Chapter I
This appears to be the only reference specifically to the "Black Pits" in The Lord of The Rings. While Gimli does mention the "Black Pit" in The Fellowship of the Ring when describing Moria,
I know them and their names, for under them lies Khazad-dûm, the Dwarrowdelf, that is now called the Black Pit, Moria in the Elvish tongue.
The Fellowship of the Ring, Book II, Chapter III
it seems that the "Black Pit" Gimli referred to is different from the "Black Pits" Shagrat mentioned because Gimli specified a singular "Black Pit" and Shagrat alluded to plural "Black Pits." Therefore, what were the "Black Pits" that the orcs referred to?