Idle blocking comes up rather often. Look over this thread here on thwack also. Why do I see an "idle blocker" as a head blocker? How can I find the cause?
Other ideas here are as mandevil says in that thread "drill into Ignite to see when the blocking had started and see what that session had been running just prior to the blocking condition."
I have also seen a few users have success with some custom reports to zero in on what was running before going idle. such as these Custom Report - Find SQLs by Session ID (SID) , Custom Report - Find SQL by SPID depending on your repository and if it is Oracle or SQL server for what one to use on those. Something I would also like to see is to get this feature in to help with finding these blockers Add information to the trend view blockers tab like the current view blockers tab.
I hope this all helps some.