Is it normal for the blue egg gene to just not show?
Yes, as you have just seen. With only one pullet that is not at all unusual.
She is just a mix so who knows what’s in there.
Since a pullet that lays brownish eggs hatched from her blue egg I can tell you. But you sure can't tell by looking.
The blue eggshell gene is dominant so if the hen has either one or two blue eggshell genes at that gene pair she will lay a blue of green egg. You can't tell by looking if it is one or two. The The alternate to blue is not blue, which defaults to base white. Brown or green is just brown on top of base white or base blue. It doesn't matter which parent has the blue or not-blue genes. Since a rooster doesn't lay eggs you can't always be sure what he has unless you know a lot about his ancestry.
If either parent has two blue genes the pullet will lay a blue or green egg. It doesn't matter what the other parent has, she will always get at least one blue eggshell gene.
If either parent has one blue eggshell gene and the other has all not-blue, about half of the pullets will lay a blue or green egg and about half will not. Graphically, if capital "O" stands for the dominant blue eggshell gene and small case "o" stands for the recessive not-blue gene
Oo +oo gives you:
50% Oo so your get a blue or green egg, and
50% oo so a brown or white egg
If both parents each have one blue and one not-blue
Oo + Oo gives you:
25% OO so a blue or green egg,
50% Oo so a blue or green egg, and
25% oo so a white or brown egg.
Since your pullet out of that blue egg is laying a brown egg she has one blue eggshell gene and one not-blue gene. That much is for sure. It's possible the father could have one blue gene but from what you said it's doubtful. So about half the pullets should lay a colored egg. But those odds are on each egg. You'd have to hatch enough for the odds to mean something.
