David, somewhere here (probably on the main bluebird site (Sialis.org) tells that this fishing line is not recommended because other birds (including Blues) can get caught in it and get hurt. The Van-Earp trap is the way to stop those HOSP. But you could give it a try until you get something better.
Yes, I just checked and it is the Sialis.org website (which has oodles of information about bluebirds) that tells about this fishing line matter (it is the very first big paragraph on their website). Good luck!