pink eyed blue (silver) does not actually show up in a rat, it looks exactly like champagne (pink eyed black) so unless you find a breeder that knows their genetics and is breeding PE with blue I don't think you will find one, I don't know of any in the UK, but you may find one that isn't NFRS registered, have no idea what the type on such a breeders rats will be like.
I wouldn't mix Russian Blue with British Blue either, you will get Russian Silver & you will wreck all the hard work you have done on Powder Blues, you will also get blotchy patchy top coats and bunching of colour on any resulting British Blues which takes ages to fix.
Unfortunately the rat fancy is unlike the mouse fancy in that very few breeders have a line that breeds true, I can think of only one or two who can completely predict that their rats will throw.
Rat breeders mix and match constantly and far too much in my opinion. Rats are not line-bred in the same way that mice are and so their genetics are much harder to predict over here & again you cannot always trust that they carry what their breeders say they do.
I've had Blues turn up in lines where is should not be possible and Siamese pop up in litters where it is not showing for 12 generations on a pedigree.
Julie is right in that a good agouti is always the best option if your goal is to improve type, however it will darken the colour initially, however as a mouse breeder you will be used to breeding by selection and should be able to get back to where you are again, it just takes longer in rats thats all
