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"Aasimar often have features that hint at their celestial heritage"
The learned adventurer would know your race. There are other Aasimar adventurers even within the guild this very night.
Though they would also know about dark aasimar, they wouldn't assume you to b e one. No evil being would be foolish enough to boldly enter the guild packed with capable adventurers.
You may have been asked to keep your race a secret by the church, they don't like competition, but that's not necessary here.