A belt sander follow by a random orbital. Got to deside early if you are shooting for a totally flat machined surface, or retain some texture of the grain/ hand finished look.
Here is a thread on a live edge slab I finished for a window desk.
With Lots of useful info from the brain trust here.
Black walnut is a beautiful wood, enjoy the project!
Cheers Richard
http://forums.pelicanparts.com/off-topic-discussions/728150-wood-finishing-questions-3.html