CE560 Captain reports encountering severe turbulence during climb after penetrating a building cell not painted on radar.

Narrative

On Feb/XA/09; we departed enroute to STL. After departure; we could see a line starting to develop to the north and decided that we would be best to deviate around the south side and requested a 090 degree heading with ATC. After flying this heading and entering a cloud layer; we were watching the radars in the aircraft (Storm Scope; WSI; NEXRAD; and aircraft radar) all of the agreeing that we were well clear of the cells to the north. While maintaining a smooth ride in IMC; ATC cleared us to a higher altitude. We broke out of the clouds in an instant to see a contrail; with no mention of turbulence above; and a building cloud in front of us. We entered the cloud with nothing else to do than to hang on. The severe turbulence knocked off headphones; glasses and cleaned out the airplane. Just as soon as I could get headphones back on; I advised ATC that we were in severe turbulence and had broken through the altitude that we were cleared to. Then I made sure the passenger onboard was not hurt; and was in contact with ATC to affirm a new altitude for cruise; he then also cleared us direct when able. A little while later; we were informed of a phone number to call when we got on the ground. Upon arrival and letting the passenger out; I walked around the aircraft to see if there was any damage; there was none; and then called the ATC desk. He also advised that their radar was not painting anything in that area and was concerned of any injuries. The gentleman also advised me that this would be filed as a miscellaneous incident since it was neither my fault nor his; just Mother Nature being mean.