If I'm not mistaken, Sean's are of the American/British Blue variety, with the locus of gg. His also looks to be a pretty powder blue. But that could change.
The American Blue line is "said" (on a couple different Rat club's) to not be as inbred and sickly as the British Blues. Of course, we could have problems in our lines that the British Blues don't have either. I think with some dedicated out breeding, you could get some healthy American/British Blues.
Mine are the Russian blue with a locus of dd.
As a pup
Now
Her pictures make her look like a dark grey, when in true light, she is a silvery steel grey, with a lighter belly.