Genuine OEM Canon PIXMA MG6420 Ink and Supplies
PGI-250 black and CLI-251 black serve different ink tanks in the MG6420. Choosing XL adds more ink, but doesn't change which tank it belongs to, so match the family name as well as the black label.
Toner Buzz carries genuine Canon PGI-250 and CLI-251 singles, XL inks, black twins and mixed packs. The CLI-251 four-color pack combines CLI-251 black with cyan, magenta and yellow; choose PGI-250 separately when you need pigment black.