Both notifications and blocking tasks are implemented over the same AMQP message bus, but they are separate mechanisms.

When a task is blocked, an extension is responsible for delivering a message to either query status of the related object or take action on the blocked task.

Resume, Progress (that was made), Abort, and Continue are valid calls against a blocking task.

A blocking task might time out. You can configure the timeout for a blocking task globally in VMware vCloud Director.

You can abort a waiting, blocking task directly from the VMware vCloud Director UI.