Diagnosis
Water is trapped in the cavity between the blade shaft seal and the bottom of the plastic pitcher base. This is common if the pitcher was submerged or run through a dishwasher.
How to Fix It
Do not submerge the pitcher. To dry it, unscrew the blade assembly by turning it clockwise (it's reverse-threaded). Remove the blade, the metal drive socket, and the rubber sealing ring. Dry all components and the pitcher cavity thoroughly with a towel. Reassemble tightly.
If That Didn't Work
The rubber sealing ring (gasket) on the blade shaft is worn or damaged. This allows water to enter the cavity during washing. It must be replaced. Ninja part #TB301GR.
Heads Up
Storing the pitcher with water inside the blade cavity can lead to mold growth and a compromised seal. Always dry it completely before reassembly. The pitcher is top-rack dishwasher safe, but disassembling the blade first is recommended.