Diagnosis
The silicone sealing ring is missing, damaged, or misaligned. This ring sits in a groove on the blade assembly.
How to Fix It
Unscrew the blade assembly from the cup. Find the clear or black silicone O-ring. Remove it. Check for nicks, tears, or flattening. Clean the groove thoroughly. Re-seat the ring evenly in its groove. It should not twist. If damaged, you must replace it. Use part RP-1 or equivalent.
If That Didn't Work
The leak path could be the cup itself. Inspect the plastic for hairline cracks, especially near the base threads. Fill the cup with water, hold it over a sink, and screw on the dry blade assembly. Look for drips.
Heads Up
Never overtighten. The seal is made by the ring, not torque. Cranking it down distorts the plastic and creates gaps.