From what I read (but can't find it anywhere) was that blue irids are the top layer of colouring on a betta so the silver dragon scales cant cover the blue...or something like that.
Anyways I found this snippet of info, I'll try and find the other piece that explained why its difficult or impossible.
" The characteristic thick, silvery/white metallic layer on the body is typically found red, yellow, orange, white and black "dragons". In case of red, yellow and orange "dragons" this layer can be observed in both light- and darkbodied fish.
Interestingly, so far this characteristic silvery/white metallic layer has not been observed in darkbodied bettas with a classical iridescentcolor (steel blue, turquoise or royal blue) of the body and finnage.The picture shows an imaginary example of what the authors mean with this.
In case of a dark bodied "dragons" with a classical iridescent base,the silvery/white appearance seems to be masked although based onmacroscopic observation (with the naked eye) their scaling definitely seems to differ from a regular metallic fish by being more solid. There have been reports of red and black dragons with a blueish shine ontheir scaling but so far no real blue "dragons" with this typical thick, silvery/white scaling have been developed. It would be interesting to find out whether it is possible to obtain a the thick,silvery/white metallic layer back after crossing a blue or copper"dragon" as depicted above to a red, yellow, orange or black betta"
Heres what a true blue dragon would look like (been photoshoped)