Darkmeyer notes are a type of currency used in Darkmeyer to trade with the Darkmeyer blood trader. You can obtain this item if you refuse to kill the captured Meiyerditch citizens in the upper tier jail. The Vyrewatch guard will blackmail the player into retrieving a debt from a Canifis settler. In order to threaten the settler to retrieve the debt, you must have a Wolfbane dagger in your inventory while speaking to them.
You can still obtain the notes if you have killed citizens, although the blood trader will still not trade with you and so they are of no use in this situation.