sp_configure 'show advanced options', 1 ;
GO
RECONFIGURE ;
GO
sp_configure 'blocked process threshold', 10 ;
GO
RECONFIGURE ;
GO

Create a new trace in Profiler and select the "Blocked Process Report" event listeds under the "Errors and Warnings" event category.

After this setup, you should see all blockings taking longer than 10 seconds using this trace.