Yes, I've seen a variance in the intensity and distribution of the blue shell pigment,Acutally, a white egg is possible from a blue egg layer... just as a darker blue can happen as well... yes, the pigment is inside the shell itself, but I have seen where it didn't get evenly distributed and was darker at one end and lighter at the other... I've also gotten an extremely pale egg I could have sworn was white, but with very close inspection it had just a bare hint of blue...
Just as brown egg layers can run out of paint, or have 'malfunctions' in the coating amounts, so can blue egg layers have pigment mixture issues...
but never truly white.