Why Blocking dynamic range measures how strong an unwanted signal can get before it starts choking off the receiver's gain. The bottom reference is the receiver's noise floor (MDS), and the top reference is the level of an off-channel signal that compresses the receiver gain by 1 dB. The span between those two levels, in dB, is the blocking dynamic range, and bigger is better because it means strong nearby signals do not desensitize the receiver.
Watch out The choice about two signals producing third-order products above the noise floor describes intermodulation-distortion dynamic range (IMD DR3), a different spec measured with two tones rather than one strong blocker.
Blocking DR = noise floor up to the 1 dB compression point. One signal, 1 dB. Two tones means IMD dynamic range instead.