I don't think painted oak will stand up better than oak with a clear finish. In fact, some of the Spar finishes will outperform all paints.
If you have your mind set on black you could paint the boards and then apply many coats of the Spar urethane over it. Keeping it up would be as simple as an occasional cleaning, sanding and fresh coat of more clear.
But then again, that is the way it would be with clear oak as well. So if you like the looks of clear oak you might as well keep it.
