DIAGNOSIS Organic residue (smoothie, milk, juice) is trapped in the seal groove, promoting bacterial and mold growth. This is a maintenance, not a malfunction, issue.
FIX Disassemble the cup after every use. Remove the blade assembly and the silicone gasket. Wash the gasket and the exposed groove in the cup with hot, soapy water. Use a small brush (toothbrush) to scrub the groove. Dry all parts thoroughly before reassembly. Do not reassemble the cup while parts are wet.
IF THAT DIDN'T WORK Persistent stains or odor may require soaking the silicone gasket in a 1:1 water and white vinegar solution for 30 minutes. Rinse thoroughly. Do not use bleach, as it can degrade the silicone.
HEADS UP Never store the blender cup with the blade and gasket assembled. Always store them separately to allow air circulation and prevent mildew formation in the seal groove.