The Silver Ameraucana line was created by John Blehm and Mike Gilbert, they introduced the pattern from Silver Leghorns(not surprisingly the APA SOP calls for the same description on the male based on the Silver Leghorn: silver hackle with black striping, and a silver saddle with no striping),
The blue eggshell gene came from the easter egger lines they crossed with silver leghorn as Mike Gilbert said so himself "easter egg chickens were crossed with other breeds to get the recognized colors".
In the case of Silver Ameraucana, logic would indicate that the original breeders used wild type based or wild type looking EEs stock that laid blue egg shells to start with(some EEs do lay blue eggs, while most lay green) as that would have saved them a few generations and Silver Leghorn stock, bred them together and refined the most important traits like blue eggs, slate shanks good type, color can be improved after that.
Silver Leghorn rooster
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Silver Ameraucana rooster
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Silver Ameraucana like Silver Leghorns are regular silver duckwing e+/e+ S/S(S/- for females), they are not silver quail pattern.