Which Monster Drops Blue Charms Frequently?
To be quite honest with you, none. Blue Charms are meant to be rare, and as such there is no magic monster which drops them “frequently”. All you can do is fight monsters which drop them the most, and in my own experience, Gargoyles and Kalphite Soldiers drop them fairly often (for Blue Charms – they will still be scarce). If you’re trying to train Summoning, give up on Blue Charms. Crimson is where you want to be if your level is getting up around and over 50. If you’re simply searching for Blue Charms to make some familiars, read above again.