You are not likely to get the weathered "black" from the oak without sanding it out. Even then it will probably be an uneven finish, if you are using a clear varnish or stain. You could use a varnish strip product first, then maybe something like a deck restoring product to see if it will remove all the "black".

I've tried to finish mine with an "oil" finish, by using stain, linseed oil and turpentine mixture. The thinking was being able to re-apply the finish whenever it got weathered. I haven't installed my bed yet so I am thinking of taking the shine off then using 00 steel wool to rub in more oil.

Good luck with how you decide to proceed.