What is a good way to get blue charms on RuneScape?

One of the fastest ways to get a lot of charms - including many blue charms - is to kill rock lobsters with the spell "ice barrage". Note that this spell requires a high magic level, and a lot of money. Otherwise, I would suggest you just do slayer tasks - preferably from the highest-level slayer master that is feasible for your combat and slayer level - and not worry about what type of charms you get; however, the genral tendency is that higher-level monsters give you better charms (for example, less gold, and more crimson and blue).