I recently upgraded Jenkins from 1.609 to 1.642 (using the stable Debian releases); when I did that, Build Blocker Plugin stopped working as expected. (The version of Build Blocker Plugin was 1.7.3 in both cases.) The problem involved two classes of jobs that I have, "Upgrade" jobs and "IT" jobs: I don't want Upgrade jobs to run at the same times as IT jobs or vice-versa, and in the case of a conflict, I want the in-progress IT jobs to finish (without launching any new IT jobs) and then the Upgrade jobs to start.

I have this configured with the Build Blocker Plugin: Upgrade jobs block on ITs, but only on running jobs, not on jobs in the queue, whereas IT jobs block on Upgrade jobs, including the jobs in the queue. This gave me the desired behavior with Jenkins 1.609; with 1.642, however, I observed newly launched IT jobs running even when Upgrade jobs were in the queue at the time when I launched those IT jobs. (Though the Upgrade jobs did seem to be blocking properly on running IT jobs.)

Looking through the Jenkins Changelog, I was suspicious of JENKINS-28926, which came in with 1.618; I've now downgraded to 1.611 (because I want the fix for JENKINS-27871, which appeared in 1.610), and blocking seems to be working as expected again.