What is this one-handed tool with five dull steel "blades" called?
It's a pastry blender, less commonly also called a pastry cutter or dough blender. The primary use is cutting (mixing) solid fat into flour to make pastry dough.

Another common variety has stiff wires instead of the metal strips yours has. 



That kind also works fine for pastry dough, but likely won't work as well for your alternative use on clumpy powders; it'll tend to flex out of the way a bit, especially if the clumps are hard.