According to the FAQ, the free version will not export to other 3D formats, but the pro version exports to quite a few 3D formats including 3DS which you can load directly into Blitz3D.
See: http://sketchup.google.com/support/bin/answer.py?answer=36203&topic=9029
See: http://sketchup.google.com/support/bin/answer.py?answer=36203&topic=9029