I have the HDRUX1 and I know exactly what you are talking about.
It seems the iris just cant open enough to compensate for the fps your camera is trying to simulate. You will need to increase the light your using for slow motion to work.
I dont really know what to say about the blurryness. Ive had the same problem when using my little camera for an engineering bridge breaking competition. Im guessing its just an codec problem. The camera may have to lower the bitrate inorder to create 240 fps. Then again, it could also just be the lack of light.