In the ebuild, bbrun depends on blackbox. However, without any of the blackbox clones install it will compile just fine, since its only real dependance is GTK+.
Most bb* apps depend on virtual/blackbox because they use special atoms provided by blackbox-based wms to integrate into the envrionment. Is this not the case with bbrun?
Works fine without a blackbox-based WM. Just tested it on my test box running only Enlightenment and it performs exactly as it does under my main box running Fluxbox. Fixed along with a version bump to 1.6. Thanks for the report.