Many Canon printers (e.g. iX6500) have two types of black ink. Suppose you just want to use one - for example, you need to print something urgently, but there's a fault with the other type of black's ink cartridge and that's the type it is trying to use.
How can you force it to use the other type?
PGBK ink is intended for uses like text, it's smudge resistant and good for fine lines. BK ink is intended for photographs, it's richer. The printer defaults to PGBK for text, lines etc - unless it thinks they are part of a photo.
So, you can force a Canon printer to use BK ink instead of PGBK by telling it that you are printing a photo onto plain paper.
Warning: as user395664 commented, doing this does substantially slow down printing speed, even on "standard" quality.
In printer settings, change paper type to "high resolution paper". Problem solved! Thanks ~ Dan Richard ~ http://www.danrichard.com/2011/07/14/how-to-use-the-other-black-ink-cartidge-when-one-is-empty-on-your-canon-pixma-mx850-printer/
On some printers (e.g. Canon TR8500), there is an option to do this under printing preferences - Page Setup / Print Options / Print With - see below:
