motion export
When I export a motion project, the background color reverted to black.
MacBook Pro, OS X Mavericks (10.9.2)
You can only know for sure if it is black or transparent if you layer it on top of something else.
If you play a movie with transparent areas in, for example, Quicktime Player, they will appear as black. That is normal.
Now if you edit in FCP X and connect your movie above some other clip, the transparent areas will let you see the content of the lower clip.
Also, in order to export with transparency, you need to use a codec that supports it - that will need to be at least ProRes 4444. Neither H264 nor ProRes 422 support transparency.
You will also need to export the alpha channel: