According to what my mechanic said, the blue one is the original factory fitted sensor, while the black one is the genuine service replacement. The Intermotor replacement is white, and (apparently) no good for the V6.
Can't really take it back as I asked for it by part no. These things are so difficult to prove that the £13 it cost may best just be seen as a lesson.
Having said that, there's also a principle involved here.