DIAGNOSIS Food and liquid seep under the removable silicone sealing ring. It's the number one source of odors and isn't dishwasher-safe.
FIX Pop the ring off after every use. Pry it from the inner edge with your thumb or a blunt tool. Hand-wash it and the groove it sits in with hot, soapy water. Let both dry completely before reassembling.
IF THAT DIDN'T WORK For set-in stains or smells, soak the ring in a 50/50 mix of white vinegar and warm water for 30 minutes, then scrub.
HEADS UP Never store the blender with the sealing ring installed while damp. Mildew will form in that dark, enclosed space.