DIAGNOSIS That black silicone ring is a prime trap for seeds, spinach bits, and protein powder. It's designed to seal the vessel, but its soft texture grabs onto debris.
FIX The best method is immediate action. Right after blending, fill the vessel halfway with warm water and a drop of dish soap. Attach it to the base and run a 10-second blend. Dump that out. Then, physically pop the silicone ring out of the lid by pushing it from the underside. Wash it separately under the faucet, using your fingernail or a soft brush to dislodge gunk.
IF THAT DIDN'T WORK For dried-on stains or smells, soak the entire lid assembly (with the ring removed) in a bowl of warm water with a tablespoon of baking soda for 30 minutes. Rinse thoroughly.
HEADS UP Never put the lid or the vessel in the dishwasher. The high heat can warp the plastic and ruin the seal of the gasket, leading to leaks.