I had recently been boiling a piece of driftwood in an anodized aluminum pot. Did quite a few boilings, and often let it sit in the pot overnight to cool.
Water was usually pretty dark when I emptied it.
The pot looks... different? Not sure how it's affected, I washed it out fairly thoroughly the other day, and The finish doesn't look the same.
I'm not terribly concerned, as it's a pot I rarely use, and when I do, I'm usually just boiling a lot of water or something. (plus, restaurants and such cook in naked aluminum all the time...).
Stainless steel should be fine, but you might want to be careful about anodized aluminum.