Considering that .pjt files are projects from a tool called Didger (geoprocessing toolbox), it is likely that pjt to blb conversion is some kind of export of the project or part of it to another format. Unfortunately, we have not found anything specific about this, beyond the fact that Didger does support export to BLN format. So if you struggle with the export, it would be perhaps best to contact their support or some online community.
Updated: July 17, 2023