The blue cube isn't the crystal. Try shooting and/or buttstomping (don't remember exactly what it was, but one of these should work. edit: just checked this: buttstomp at the bottom of the right path under the blue block, then shoot the blocks on your right, then buttstomp the other blue block) in the area below it, and you'll get access to another room, where you should find a "multicolored tzarin gem" (or something like that) which is the thing you should destroy. If you go to the part that mentions destroying the crystal now, you'll still get locked in, but the part behind it, called "red rock cave", will be open now.