It seems that when you enable "Block build when downstream project is building" that scheduled polling still occurs. If you use fstrigger and a poll sees a file change when builds are blocked due to running downstream jobs, the timestamp or "fingerprint" or the file/folder watched still gets updated. This means that once the downstream job completes and the block is removed a new job should be triggered. Currently builds are NOT triggered as the plugin is comparing the contents observed during the blockout to current which are the same.
When builds are blocked fstrigger should not update its "state" used for comparisons.