I think it's because the black turns kinda grey when you put it on white paper.. So to get the true shiny black, HP will use a little color on black as well.. If you want to print using just black, then you turn on "print in greyscale" in your printer options.. I have an HP deskjet that also uses color when it prints just black text.. Really sucks, but I guess that's just HP lining it's pockets full of money on those color ink carts, but there *might* be a real reason behind it as well..
In any case, printing in greyscale really uses only the black but I noticed it's no longer a shiny black, at least on the ink jets.. It's more of a really dull black without mixing some color.
Onboard RAID vs. 3Ware RAIDI never recommend people run RAID-5 with onboard chipsets.