From the dim and distant past I believe that in the early days of Radar the materials available to manufacture radomes were not very receptive to painting this meant that they got to look very tatty in no time at all so it was preferable to leave them black. I think that there's mention of this in the history of The RAE by Reg Turnhill (I can't remember the correct title) A lot of work was done on this at Pendine, firing samples through water sprays on the rocket track in the sixties and early seventies. Originally I believe to allow anti-flash coatings to be applied to the V force however by the time the new coatings were ready for use the V bombers were moved to the low level role and they left the radomes black!