I bought about 2K 178gr Amax about 5yrs ago. And to make a long story short, when I started loading for the 308's again, low and behold, the bullets were tarnished just as yours. I simply take a hundred when I'm ready to load and throw in the ol trusty Tumbles Tumbler (Corn Cob & a little Dillion Polish) for a couple of hrs and they come out looking bran new. Doesn't affect the poly tip or anything.
Just a guess? Are these Hornady's? I have never seen bullets from any other manufacturer when a sealed or taped box is opened, to have tarnish, but sometimes Hornady's will. And, (happily) it don't hurt a damned thing, I wouldn't worry about it. BB