To do a b3d exporter for Blender your best bet is probably to get Peter Scheutz's XML version of the file format from the Code Archives. It is easy to write to a text file from Python, and then use his code to convert the text data into a b3d file.
This is the approach I'm taking working on a Maya exporter: use MEL to write the xml file, and then use his code to convert to b3d.
This is the approach I'm taking working on a Maya exporter: use MEL to write the xml file, and then use his code to convert to b3d.