Generally I've seen that the blocked stack port is between slot 1 port 2 and slot 2 port 1, although having a link down or 'no-neighbor' in 'show stacking stack-ports' may affect this behavior.
Checking in the EXOS User Guide, I see no reference of how this stack-port is determined, and I see/know of no way to manipulate this behavior.
Note that the 'blocking' only blocks Broadcast, Multicast, and Unknown Unicast flooding. Normal Unicast traffic still traverses the blocked stack port to use the shortest path to the destination slot. Ultimately, which port is blocked shouldn't have an significant impact on stack behavior, although a flaky link can cause stack-instability depending on location.
Hopefully that helps!