Is there a particular reason for a Designated Port to continue sending out BPDUs after convergence has occurred on a link where the other side is in a STP blocking state? I get the idea of a Designated Port when a switch directly connects to the Root Port of another switch or when connecting to an edge device like a PC, but what's the purpose of determining a Designated Port between two switches when one side will inevitably enter into a blocking state to prevent a loop? Why don't both sides enter into a blocking state? Assigning a Designated Port in this case seems irrelevant. Is this just how the STP algorithm works?
BPDUs are still sent because the switch topology could change, and it will then be detected. For example, if the root port on the switch with the blocked port either goes down or loses a path to the root in some other way, it will then look at the BPDUs being received on the blocked port and may decide that will now be its new root port.
Once STP has converged, that does not mean that things cannot change, otherwise you would need to restart everything when a change happens, or you could actually create a loop by making a change. BPDUs are continued to be sent to detect and adjust to any changes in the switch topology.