The "Build the Vault" instruction for Adventure 7-3 reads:
Build the Vault: Start with all level 0, 1, 2, 3 cards from the Core Set and the Curse of the Crimson Throne Adventure Path, then remove all Level 0 non-Veteran banes and you may remove any Level 0 non-Veteran boons.
Notably, it does not say "non-Veteran, non-Blessing boons" as most higher-level Build the Vault instructions do.
Is this an oversight or are we intended to be allowed to remove the Level 0 Blessings?
This pretty much matches the Adventure 3 instructions for Curse of the Crimson Throne:
"Upgrade the Vault: Remove all level 0 non-veteran monsters and barriers and you may remove any number of level 0 non-Veteran boons. Add the level 3 cards."
No mention of the boons being "non-blessing". This is reasonable, since you have ample level 1 blessings from Curse and Core (which are Veteran, so they'll always be there).
I'd guess it's intentional.