This alert will run against the monitored instance and retrieve the time each session has been blocked ("Seconds Blocked" column). It will also give some details about the blocking session in the "Message" column and this can be customized with anything else from sys.dm_exec_requests DMV.
To create the alert, click on Alerts > Manage Alerts tab and create a Custom Alert of type Custom SQL Alert - Multiple Numeric Return. Configure the alert with values similar to these:
Alert Name: Blocked Sessions
Execution Interval: <= 10 minutes down to 1 minute
Notification Text: These are sessions being blocked.
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