Digital BW, The Print

I just saw an episode on Modern Marvels (awesome show!)about Iron.

In that show, they showed one of the big uses of iron is in the oxide
form, for pigments.... and that black iron oxide (the black caught my
attention) is used because of its longevity and blackness, and has
been used for centuries by artists and is used in to print the black
in US paper currency. HELLO! Isn't that exactly what is needed for our
BW printing?  :-)

Just curious - how come we battle trying to make carbon inks look
black when, apparently, there is a better black available - and
long-lived too!