>>Well, it could be a number of things. First off, black rats that are exposed to a lot of light (be it sunlight or room light) will fade (more often into a chocolatey-black color).
Some black rats also molt into this color.
Ok those are things I've seen before with adult black rats & I don't think this is it.
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>>Pet-quality (not show quality) rats tend to have what is called "silvering". It can make them look greyish.
I don't see any silvering which in my understanding is lighter colored hairs mixed in or on top of the colored fur. I see that in the blue healer that lives next door but it doesn't seem to be what I saw on the rat.
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>>The last possibility, which is the least likely (considering you probably didn't pay a breeder for a rat that they couldn't tell you what it was... and these are much rarer than most people know) is that you have a Russian Blue. They are a very dark blue, but it would still be obvious that it wasn't a black rat.
I'm thinking this is the one because I suspect the rat came from Pet's Mart & I've seen this color in their rats in the past. The supplier they get rats from has sent in some gorgeously colored rats and still does do that- they just aren't healthy rats unfortunately.
>>Oh, and one more thing: is it young? Young black rats sometimes appear lighter than black. It changes once they molt their fur though.
Age I don't know but I suspect fairly young, probably less than a year old based on size of the rat but.. on the other hand the rat has very orange teeth, not something I see that often in younger rats. Usually they're older when the teeth turn color like this. This one though had been fed a crummy diet so I don't know if that has anything to do with the color of the teeth.
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>>That's my two-cents
Your 2 cents are very helpful, I may be wrong it may not be russian blue at all but from what I know & what you've said it may be. Oh well, it's not that important either way I was just trying to satisfy my own curiosity.
Thanks for helping with that. 
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PHRatz