What does blocking performance mean in RF transceivers' datasheet?
Blocking is the degree to which the sensitivity is reduced by the presence of high-power signals adjacent to the band of interest.

A quick googling found this, and this: 


  Blocking is defined as the degradation of receiver sensitivity, in the presence of a much stronger (blocking) signal.
  Therefore we must first understand that Sensitivity is the measure of
  what the smallest possible detectable signal is.
  
  Let's assume you have a radio receiver and you monitor a certain
  signal at an SNR of 6 dB. Now if a much stronger signal was present
  near your signal, the SNR of your desired signal would now be less
  than 6 dB.
  
  Unfortunately for us, in all radio systems, the blocking signal does
  not have to be masking the desired signal, as is commonly used in
  electronic warfare. Blocking can also be caused by regular high power
  radio signals spectrally near your desired signal.
  
  The simplest method for this kind of blocking to occur would be if the
  interfering signal was strong enough causing the receiver LNA (the
  first amplifier in the receiver) to reach its compression point, thus
  causing a decrease in gain and potentially raising the amplifier's
  noise figure. 
  
  Due to this, "blocking dynamic range" is often defined as "the
  difference in dB between the level of an incoming signal which will
  cause 1 dB of gain compression, and the level of the noise floor".