One of the two bridges will be root (the one with the lowest bridge ID). The blocked port will be on the non-root. The non-root bridge receives bpdus from the root on two ports. It will elect the port that receives the best bpdu as root port (forwarding), while the other will be elected alternate port (discarding).

The parameter in a bpdu are: root bridge id, root path cost, sender bridge ID and eventually sender port ID. As the two bpdus are coming from the same root bridge and the links have same cost, the 3 first parameters are the same. The sender port ID is the tie breaker to decide which bpdu is best. So... the answer is:

The blocking port is the port on the non-root bridge that receives the bpdu transmitted by the port of the root bridge that has the highest port ID. Note that the port ID is made of a port priority plus a port number (the port number is not the only parameter).

Hope this helps;-)

Francois