I'm pretty sure the blackening effect is just a simple oxidation reaction. My aluminum pots always darkened with use. And when anyone used a aluminum-shafted paddle during a day on the river, they always had black hands. Sure, it's a nuisance, but I sincerely doubt it's in the top 20 things that will kill you.

BTW, anybody who uses anodized/hardened aluminum pots: does this have the same effect on the flavour of food/tea/coffee that the old aluminum pots used to have? Very interested in your experiences.