They're weak.
They deal weak normal damage, have no ranged attack like status jellies (either shooting spines or ground spikes), etc. Okay, I get that, it's a blast cube, it's special. However, don't really know if you guys find the "blast" or explosion from the jellies' deaths really a threat or hurts much. I sure don't. I guess the damage is not that weak, but they (the jellies' waste droppings) are so easy to dodge. One could as easily as to just sit by the purple thing and wait for the counter, get the loot and run away. Surely, that's not how an angry "explosive" works.
Suggestions (different suggestions, not combined):
-Make it have a random, 50/50 chance of exploding like Tier 1 Blast Cubes upon detonation and destroy those snuggling close to the sticky purple goo. (Not very hu... err knightitarian, I know), or to drop out those droppings.
-Allow the droppings to follow the closest, unsuspecting knight and give him what he deserves.
-Becomes red like Tier 2, and is like a mini-Jelly King gone berserk. Does large twirl (the attack that it does, not like... a dance or something)
Just another idea.
I kinda feel like T2 and T3 blast cubes need to have the same explosion as the T1 blast cubes AND the jelly thingies. Maybe the T3 jelly thingies can have the 3x3 explosion like an explosive block? Right now they're so predictable and the explosion is so tiny that it's utterly inconsequential. Maybe the T3 jelly things spread out much further so they're a threat in a larger area, and have a larger explosion radius (like I mentioned before).