Rats can be recessive for a truck load of genes. Just because you don't think something is present in your colony, doesn't mean the genetics aren't there.

Honestly, judging by nothing other than appearance, I'd say a dilution of Platinum. Platinums can either be produced from a Mink x Blue or Blue x Red Eyed Dilute.

There are also Silvers, which are a Blue x Pink Eyed Dilute.