Good observation - do you look at the inside color when the egg is freshly cracked open? Once the membrane dries it looks stark white. If you can peel membrane off sometimes you see a bit different color.Are the inside of the eggs also blue or are they white with blue outside? I have some that are off blue/slight light green. They are white inside with blue outside. I have two that have blue inside and outside. I of course are going to simply breed the blue insideout this year.
Since you are going to use the ones that you have that are blue in and outside -- I guess you have already solved it. Technically my understanding is that the blue pigment is throughout the shell.