I'm not sure if the drop rates have changed or anything but when I did Slayer tasks a while back, I noticed I got a good amount of my blue charms from Gargoyle tasks.
I'd train slayer, I have gotten mass amounts of charms throughout slaying, as well as herbs. I get massive amounts of blue charms from Dark Beasts, Gargoyles , Iron Dragons, and Tormented Wraiths.
I would go with iron dragons. Blue Charms are common for iron dragons, and you have a slim chance of getting a Draconic Visage or dragon items, why not take that approach? Or Dark Beasts, giving you a chance to get some dark bows and a good sum of blue charms.