DIAGNOSIS: The blade assembly's sealing ring (the black gasket) is either worn, cracked, or not seated properly, allowing liquid to seep past the blades into the motor base.
FIX: Unplug the blender. Unscrew the blade assembly from the jar by turning it counter-clockwise. Remove the black rubber O-ring gasket, wash it, and inspect for cracks or tears. If it looks good, re-seat it perfectly flat in its groove on the jar threads, then screw the blade assembly back on tightly by hand—don't use tools.
IF THAT DIDN'T WORK: The jar itself may have a hairline crack near the bottom threads. Fill the empty jar with water, hold it over the sink, and check for a slow drip from the base. If it's cracked, you need a replacement jar.
HEADS UP: Always hand-tighten the blade assembly. Over-tightening with tools can warp the plastic threads or the gasket, causing the very leak you're trying to prevent.