Wall of text warning.
I've been thinking about this for a while, and this is my blue dragon genetics theorycrafting.
1) Dragons are basically metallics with "dragon" gene/genes that gives them thicker body scales/coloration. Every dragon scale color corresponds to a metallic: copper, metallic royal blue, metallic turquoise, metallic steel blue, gold, etc. The white scales of "red dragons" correspond to platinum/opaque whites (online articles were ambiguous, so I'm not sure which).
2) Platinum and opaque whites are both based on the steel blue iridescent gene. If I understand correctly, opaque whites express steel blue with the opaque gene (Bl Op), and platinums whites express metallic + with steel blue (Bl +). (I am ignoring the rest of their genotype. The steel blue is the important part)
3) All blues are expressed in the iridescent layer. Thus, any white scaled dragons can only have BlBl (steel blue) as their genotype.
Thus, I believe that any true blue dragon with white scales can only have steel blue finnage.
Note that blue cambodians are not true cambodians with a pigment-less body, but basically pastels (opaques with less/no opaque), all of which express blue/green iridescence. I think this is probably the best place to start for any blue dragon project.
If you can work in the metallic + gene, "dragon" gene (generally dominant), and opaque gene (not sure if necessary) into a steel-blue "blue cambodian" line, I'm pretty sure you would end up pretty close to a true steel-blue dragon.
My 2 cents. I'm not sure if my analysis is accurate since there was a ton of information to go through, but if anyone else has any thoughts I'd be glad to hear.