Look closely at those “blue” eggs from the crosses. Are they exactly the same shade of blue as the “Ameraucana” egg? You might look at the inside versus the outside of the shell. Some differences in color can be subtle. It just depends on what the genetics are in the mix. I’ve had some pretty blue-looking eggs from a rooster from a very light brown egg over an Ameraucana hen too but they are not the same shade of blue as the mother laid. Pretty close though.
Silkies are often crosses. It’s very possible that Silkie has mixed genetics as far as egg shell color as well as everything else. If she is laying a lightly tinted egg it’s very possible there is not much brown in the mix to start with.
Besides your question was blue + no brown = white, not different shades of blue. Leave it to a woman to change the subject!
Are you saying the Ameraucana x leghorn crosses are laying white eggs? Then some Ameraucana were not pure for blue but were split blue/white. Or are you saying they are crossing the offspring of the Ameraucana/Leghorn crosses and some of the offspring are laying white eggs? Then ¼ of the offspring should lay white eggs.