Compatible Brother LC21Bk Black Ink Cartridge (LC21Bk). Swift Ink is the best internet source for compatible Brother cartridges. Our high-quality and low prices for Compatible Brother LC21Bk Black Ink Cartridge will save you money with every print. Our LC21Bk cartridges are guaranteed to work in your Brother printer. Our high-quality compatible Brother cartridges are fully guaranteed and environmentally friendly, giving you the best print quality every time. We have the lowest price on compatible and remanufactured Brother ink cartridges and toner cartridges.