Just thought I would add this second photo that's not as close - lots of small black spots and then the big smudges of black around the screws.
I sure am hoping that someone can help us!

Looks to me like the fasteners are rusting.

Did you ever get the spots to go away? We have the same issue with out unstained treated deck. We were hoping to stain it this fall but cannot get rid of the spots. Deck cleaner does nothing. I've even made my own cleaner and increased the bleach in the mix to no avail.

It's been odd. The black marks seem to come and go - never totally away, but they lessen, sometimes to almost nothing, and then come back. I am glad to say, there's less blackening now than there was soon after the deck was built - when the photos were taken.

We didn't do anything to the deck this fall. Cold weather hit before we felt we should stain. So, hoping that this coming spring we'll be able to go through the steps of cleaning and preparing the deck for a good staining.

Sure would appreciate you writing back as you figure out what's best to do with your deck. Thanks for writing!