Your printer comes with Matte Black and Photo Black ink cartridges. Although both must be installed for the printer to work, the printer can use only one type of ink at a time. You can check the LCD screen to find out which type of black ink is currently in use:
The printer will switch ink automatically before printing, according to the type of media you select in the printer software. This consumes a certain amount of ink, so you should avoid having the printer switch ink more often than necessary.