Why two black cartridges - Canon PIXMA ip4500

[kafka]

I am a bit embarrassed to be making this post as I suspect the information I am looking for will turn out to be obvious.

I have just set up my Canon PIXMA ip4500 and it works fine, however I have not managed to find any info in the help manuals to tell me why I have two black ink cartridges, nor to tell me how to use one rather than the other. Can anyone advise?

[Dusty]

Quote extracted from here..

The ink mix benefits output quality. Dye-based cyan, magenta, yellow, and black offer a broad palette; a second, pigment-based black looks darker and lasts longer. Each ink's separate tank rests in a clearly visible slot above a permanent printhead. As other Canon printers have had for a while, an LED on the cartridge lights red when it's properly inserted--an odd color for affirmative feedback, but still informative. The Pixma iP4500's dye-based black costs 3.1 cents per page; a page with cyan, magenta, and yellow as well costs 8.4 cents. The pigment-based black, used only for plain text, lasts 5475 pages, or just a fraction of a penny per page.