This article describes what it means when idle blockers are visible in the Blockers tab in DPA.
Idle blocking occurs when you have a session that opened a transaction (establishing a lock on a resource) and then did not commit or rollback explicitly. The transaction stays open, even though no work is currently being done. Imagine you issue a begin transaction statement and then issue an update to a table. The results will come back, but you haven't closed the transaction yet. In another session, query against the table used in the first session, it will just spin until the lock has been released. The second session will start to accrue wait time in the LCK wait type and the scenario will show as idle blocker in the blocking tab.
To find what that idle blocking session was doing, you will probably have to drill into DPA to see when the blocking had started and see what that session had been running just prior to the blocking condition. There are reasons why DPA may not have gotten what the blocker SPID was doing at the time and sometimes it’s difficult to determine exactly when the blocking session actually ran their transaction. We poll once a second for active sessions. If the transaction statement ran very quickly and happened between polls we may have missed it. We don’t leverage tracing.