I would add a sanity check to avoid time-travel in blocks.

Furthermore it might be a good idea to set a timelimit for blocktransfers, a transferless connecttion of more than an hour is not likely to be a real transfer. There could be a re-positioning trip in between.

The ambiguity of block_id in GTFS was also cause for debate with the OTP and GTFS community: 
https://groups.google.com/forum/#!searchin/opentripplanner-dev/interlining/opentripplanner-dev/gywq5Z_7Id0/A4J2XZkTPGQJ
https://groups.google.com/forum/#!search/interlining$20gtfs/gtfs-changes/T5QfhkW3Ljw/DsAWK0rWz7QJ