With enough H2O2, Gold will dissolve into the 'AP' leach. This gold will then cement out on remaining copper as a black powder. Have you checked the solution with Stannous Chloride to make sure you do still have Gold in solution ?
It is usually advised to not add any H2O2 into the C2C leach, just use HCl and an air bubbler, it takes a little longer to get started, but almost no chance of dissolving gold into solution.
How many of those boards did you process ? Those black particles don't look like cement Gold, could be carbon as you suggest.
Edit - I just noticed your post count is quite high, so I assume you already knew everything above in this post.