How to clean decking - never oil/stained

Any ideas on how to cleaning decking that was never stained? The guy that sold it to me recomended not oiling it for a year to let it weather?
Anyway power washed it (since read not recomended) but still has black-ish marks on the main traffic areas -
Any ideas ?

We've used a proprietary deck cleaning liquid plus elbow grease, a deck brush and water. That worked fine. Now we have a power washer with a deck-brush attachment - used this last year in conjunction with the above and results were even better (less elbow grease too). If you want any of the brand names, let me know.