Note, the directory name as given won't work (Moodle chokes for some reason). If you rename to blackboard_6, it works fine, just tested on Moodle 1.5 ALPHA (2005051500) from last night's download.
Edit: I updated CVS, now there is a folder inside the blackboard_6_pool_import directory in CVS called blackboard_6, which can be dropped into your quiz import formats folder and should work nicely.