To an extent it depends on what one means by "blocking". I would classify those as commands which are expecting input and will wait until that input arrives, preventing other commands for executing until that occurs. Off-hand, the non-timeout versions of -
SERIN, SERXD, IRIN, INFRAIN, INFRAIN2, KBIN, KEYIN, HSERIN for non-M2
A slightly looser definition of "blocking" would include those commands which take an appreciably long time to execute but which do complete and continue of their own accord.