The http://docs.jboss.org/jbossas/jboss4guide/r5/html/ch7.chapt.html says:
BlockingTimeoutMillis: This attribute indicates the maximum time to block while waiting for a connection before throwing an exception. Note that this blocks only while waiting for a permit for a connection, and will never throw an exception if creating a new connection takes an inordinately long time.
I got a good answer at http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3954741#3954741