Blue charms are used in the summoning skill to create familiar pouches and are considered to be the most optimal choice for training summoning due to their corresponding pouches' low gold piece:experience ratio. Unfortunately, there are very few monsters that have a high blue charm drop rate.
Can be automatically picked up by the Charming imp, or converted to 16 Summoning experience points if the Charming imp is configured to do so. (This is not recommended as making pouches yields far more experience)
Blue charms are the most rare charm to get from monsters.