If you're using Piezography ink in this 9880 printer, it doesn't matter if the printer is in MK or PK mode.
What you experienced is caused by the 7880/9880 cartridge chips being programmed with all color position information (so the chips aren't color position specific, and can work in any slot). This works great, but for some reason the PK info is stronger than MK, so if the printer is in PK mode, there's no problem, but occasionally when a printer is in MK mode, the printer gets confused when reading the chip. If the printer was in PK mode, and you installed the "MK" cart (with refill chip attached), the printer would have no problem accepting the cartridge, even though the cartridge label says MK, the printer reads the PK info on the chip, and is satisfied.
To check the black ink mode, you can select black ink change, and I believe it'll list the black you're currently on (just back out at this point, and don't continue).
The solution to this is to use an Epson MK chip, and reset it when the ink level gets low (yes, our chip resetter will reset an Epson 7880/9880 chip)- or do a black ink change to get the printer in PK mode, then it'll accept the "MK" cartridge with refill chip attached.
We have several 7880/9880 printers in our studio, and use them as our main Piezography printers (they're workhorses!), they're all in PK mode and use our refillable carts with various Piezography ink tones.
I am working on updating the cartridge instructions, and will add a note explaining this MK/PK chip situation.
I hope this helps.
Please let me know if you have further questions or there's anything else I can help you with.
Best regards and happy printing~ Dana