Stony Silence stops ALL activated abilities of artifacts, including mana abilities. This is in contrast to Pithing Needle, which has an explicit exception for mana abilites.
Tag: Darksteel Citadel
Blood Moon doesn’t remove the Artifact type from Darksteel Citadel.
Blood Moon is pretty powerful, but it can't do everything. While it can make a Darksteel Citadel stop being indestructible, it can't stop it from being an artifact!