AWR Detection RNAV
By definition, clear air turbulence has nothing for the radar pulses to bounce off, so it is not detectable - the radar pulses simply pass through the moving air. This is one of the reasons why turbulence must be reported, as you will see in Comms.
Conversely, cumulonimbus has many large water droplets, and is the most easily detectable phenomenon. Where there is significant activity, clouds may "block" radar from getting to areas beyond them resulting in no reflections - this will result in a black area there, but does not mean that they are free from cloud.