My idea about it is that SD BLOCKING and TCH BLOCKING may be caused by many reasons. It can be optimization issue(such as improper definition of parameters, interference and some other RF stuffs),hardware problem such as faulty TRXs or can be also congestion as well.
SD and TCH congestion however is due to failure to assign a certain channel (SDCCH or TCH)mainly because theres no available channel for assignment.
On the level of congestion however, if TCH and SDCCH BLOCK is due to Congestion, then at this point BLOCKING and CONGESTION are just similar. Hope you get my point.