Cleaning a composite deck w bleach

Hi Everybody,
I was wondering if annyone else may have run into this. I have a deck built out of a composite mat'l called boardwalk by certianteed the siding people. It was dirty so I pressure washed it....I friend of mine told me it would be easier to clean it with bleach and to my suprise it cleaned up very nicely....but a month later the black stuff? in the porosity of the mat'l was back. I just bleached it again,,,basically just mopped it with striaght bleach and it cleaned up like new. I'm wondering if it will come back agian and if anyone has any idea why it came back. Anyone run into this. btw,,, no trees over the deck.
Thanks for your ideas, Bob
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You might consider staining , sealing and/or painting the deck. My neighbor has the same problem, seems to be happening where most of the rain runs off the roof.
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