Diagnosis
That's a common frustration. Food and oils get pushed up under the rubber sealing ring during blending and harden there, leading to potential mold and bad smells.
How to Fix It
1) Unscrew the blade assembly from the cup. 2) Use a butter knife or a small flathead screwdriver to gently pry the black rubber gasket out of its groove. Be careful not to tear it. 3) Wash both the plastic blade base and the rubber gasket separately in hot, soapy water. A small brush (like a clean toothbrush) is perfect for scrubbing the groove and the gasket. 4) Let everything air-dry completely before popping the gasket back into its groove.
If That Didn't Work
For stubborn, caked-on residue, soak the whole blade assembly (with the gasket removed) in a bowl of warm water with a splash of white vinegar or a denture cleaning tablet for 30 minutes. That should loosen everything up.
Heads Up
Never put the blade assembly in the dishwasher. The intense heat and jet spray can force debris deeper under the seal and warp the plastic. Hand-washing is the only reliable way. Do this cleaning weekly if you use it daily.