They will explode when Dust gets near them, causing high damage and burning. Fidget's projectiles can only hurt them while they are inhaling. Getting near them is quicker, but more dangerous, especially since one explosion can push Dust down toward others. Getting close and then dashing back is a quick and fairly easy way of dealing with them.
Beware: high level Blombs can cause up to 400 damage per burn, so if your defense is low, think twice as burning isn't your only problem.
Blombs drop only about 2 to 3 coins per death, making them poor candidates for earning money, and have no other drops.