Understanding Stains On Stamps -- Are They Safe?

Hi --

When I look in online stamp shops sometimes I see stamps with dark (stains?) -- especially on the perforations. I'd like to know if these are safe to buy and add to my collection -- is this just staining that happens -- or is this a sign of mold infestation.

Can you look at these pictures (the only two I could find in my entire collection that showed this staining) and tell me what you think causes these stains and if they are a warning sign as to mold, etc. Thank you!

Dark on the perforations
(notice dark edges all around):


Dark on the perforations and on the body:

mailed by a smoker of non-filtered cigarettes.... yuck!

Or even worse, foxing.