Draconic Archivist( Heroes of Horror, p. 122)
In addition to your studies of the darkness, you have spent time studying dragons and constructs.
You can use your dark knowledge ability on dragons and constructs. You use Knowledge (arcana) for dark knowledge checks regarding these two creature types.
Without this feat, a character with dark knowledge can use that class feature only on aberrations, elementals, magical beasts, outsiders, and undead.