If it was a berried female, there's a small chance it may become slightly lighter in color after the eggs have hatched. But regardless of gender, if the shrimp has become blue, I highly doubt it will change/or change completely blonde again. Most likely they are just late bloomers since most culls are sold before they become adults to prevent them from mating with the other shrimp in the tank. It is similar to the blue bolt shrimp nowadays because of all the selective breeding aiming for extreme blue bolts; some young low grade blue bolts(culls) with low % of blue coverage and intensity have higher % of blue coverage and intensity when mature.
If the line you purchased from is good, then the offspring of your culls will produce some good blues as well. Just because you bought all culls does not mean you will be stuck with all cull-grade shrimp with future generations. As Aaronious already mentioned, it is about selective breeding and how good the genes are from your source. If the line from your source is already good, you simply have a head start at achieving a true blue strain(or blonde if that is your preference, then purchase from a "terrible" source) with selective breeding.