DIAGNOSIS Frozen residue has cemented the blade threads to the lid's drive socket. This happens if the assembly is put away wet or not cleaned promptly.
FIX Soak the entire lid assembly (with blade attached) in a bowl of warm, soapy water for 20-30 minutes. This will soften the dried-on mix. Remove, dry the exterior of the lid for grip, and firmly grasp the silver blade hub. Twist counter-clockwise with steady pressure. Do not twist the plastic wings of the lid.
IF THAT DIDN'T WORK Use a hairdryer on a low heat setting to gently warm the plastic around the blade shaft for 60-90 seconds. The expansion can break the bond. Wear an oven mitt, then try twisting again.
HEADS UP Never use excessive force or tools like pliers on the plastic blade. You will crack it. Always fully disassemble, clean, and dry the blade and lid after every use to prevent this.