We frequently get alerts for a "Blocking process" where 1 process is blocking itself because it is running in parallel and the threads are blocking each other. While this might represent a scenario that can be improved this is normal activity in queries that use a parallel execution plan. This alert is identifiable by "Number of blocked descendants: Unknown" and the lack of any blocked sessions.

I do not think this type of blocking should be ignored. But, there is no customize the behavior of this alert. If you exclude specific queries from the "Blocking process" alert you would not get an alert when the query is actually blocked.

I recommend adding a new alert type, e.g. Parallel Blocking. This would allow for better triage of alert emails. It would also allow excluding specific queries in the Parallel Blocking alert and still receive other Blocking process alerts.