This metric reports the number of connections that are currently blocked, divided by the total count of current connections.
Most well-balanced SQL Servers will have some degree of blocking. This metric attempts to measure the impact of lead blocking queries against other queries. Higher values indicate that many connections are being blocked, and queries should be tuned to reduce the amount of contention.
In this tip, Basit Farooq shares a query written using dynamic management views (DMVs) that will help you to quickly identify SPIDs and other useful information about the processes that are causing blocking on a SQL Server instance.
When excessive blocking occurs in SQL, performance degrades. Learn how to use the Blocked Process Report to rapidly resolve excessive blocking scenarios in SQL Server 2005/2008.