After getting my printhead to a state where it seems to work okay, I find there's still a very faint white line in the black block when I print a nozzle check. Rather than doing a bunch of head cleanings that waste other colors, I thought I'd just print some sheets of black ink. When you put a drop of the black ink in water, it appears to actually be very dark purple, but I assume it still contains more black than whatever other colors are in it. Does anyone know what colors Canon actually uses to print "black"?