How much darker? Is it harder on the outside? I've had mine be darker but after it warmed up it was fine.
I'm not sure but I think that means it's been exposed to air. I think it may be rancid. Here is a link about it.
http://answers.yahoo.com/question/index?qid=1006040513267
Oh wait. you said it was new. I don't think anything would be wrong with it then. maybe see how it gets after sitting at room temerature?
Well........I have seen this before, but I have used the butter and had no problems. I think it occurs because it was warmer at one time and then quickly chilled, which causes the outer "layer" (for lack of a better term) to form ... sort of like a "crust" on your buttercream, and, yes, it looks just a bit darker.
I don't think salted v unsalted has any impact here. I've seen it happen to both kinds.
I would use it if it were me, especially if you don't smell any malodorous vapor coming off the butter.
I don't know if this was helpful enough to you, but I sure hope so!
Good Luck,
Odessa
It is not necessarily that it is rancid, just that it has been sitting around for a while. The water in the butter (yes, there is water there) is evaporating, leaving the fat behind. The fat is hardening, looking like yellow crisco, right?
As long as it does not smell or taste sour, it will be fine. You can use it in baking just as you would anything else. Just cream it a little longer to break up the pieces.
Theresa 
It is not necessarily that it is rancid, just that it has been sitting around for a while. The water in the butter (yes, there is water there) is evaporating, leaving the fat behind. The fat is hardening, looking like yellow crisco, right?
As long as it does not smell or taste sour, it will be fine. You can use it in baking just as you would anything else. Just cream it a little longer to break up the pieces.
Theresa
Theresa, thanks for the excellent explanation! I didn't have time to look up the possible reason in my books when I went home for lunch and answered this thread, but now I recall reading that exact thing. I knew it was somewhere in my head!
We such smart cake people 
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