DIAGNOSIS The leak is almost certainly from the blade assembly seal. Either the silicone gasket is missing/damaged, or the blade unit isn't screwed on tightly to the pitcher.
FIX Unscrew the blade assembly (turn counterclockwise). Confirm the clear silicone gasket is seated evenly in its groove on the blade unit, not pinched or twisted. Hand-tighten the blade assembly back onto the pitcher firmly—don't use tools, but get it good and snug.
IF THAT DIDN'T WORK Inspect the plastic threading on the pitcher or blade unit for hairline cracks. A cracked pitcher must be replaced.
HEADS UP Always check the gasket is present and clean before assembling. A missing gasket will cause an instant, dramatic leak.