DIAGNOSIS Food particles and oils are trapped in the tiny gap of the black silicone gasket/rubber seal that sits under the blade assembly, and they've started to mildew. It's a notorious trap.
FIX You have to disassemble it. Unscrew the blade from the cup. Pry the silicone seal out carefully with your fingernail or a dull butter knife. Soak the seal and the blade assembly in a mix of hot water, dish soap, and a splash of white vinegar for 20 minutes. Scrub the seal thoroughly, rinse, and let it dry completely before reassembling.
IF THAT DIDN'T WORK For a persistent smell, make a paste of baking soda and water and scrub it into the seal, let it sit for an hour, then rinse. You can also soak the parts in a diluted bleach solution (1 tbsp bleach per gallon of water) for a few minutes, but rinse EXTREMELY well afterward.
HEADS UP You must take this seal out and clean it regularly, not just rinse the whole blade unit. If you don't, it will eventually ruin the smell of every smoothie you make.