DIAGNOSIS This is common. Food residue (especially starchy or sugary stuff) acts like glue in the threads, and the rubber sealing ring can get compressed and grip tightly after a hot blend.
FIX Don't use tools that can crack the plastic. 1) Fill the pitcher with warm, soapy water and let it sit for 10-15 minutes to loosen things up. 2) Empty it. 3) Grip the pitcher firmly with one hand. With your other hand, grip the blade assembly's black plastic center (not the blades!) and turn it counter-clockwise with steady, firm pressure. It should break free. If it's still stuck, try running the pitcher under hot tap water to expand the plastic slightly.
IF THAT DIDN'T WORK You might need more leverage. Put on a rubber glove or use a wide rubber band wrapped around the black plastic center of the blade assembly. This gives you a much better, non-slip grip to apply the twisting force you need.
HEADS UP To prevent this, always unscrew and rinse the blade assembly right after you're done blending, before anything has a chance to dry and harden. And when you screw it back on, hand-tight is plenty—cranking it down super hard is what makes it so tough to remove later.