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Could be soot from a nearby truckstop, smokestack, airport traffic pattern?
Could also be living crud but having it on the deck could also just mean it was in the shade---not that the deck was leaking. If you want to check for deck leaks, that's easy enough to do, and might be worth doing in any case vs. price.
Mold and mildew CAN be beaten, but if it gets in behind things and into inaccessible places, that can be a real chore.