Answer: Based on your description I am not certain what is happening here. Bleach generally will usually remove mold or mildew from a hard surface, so if that did not remove it then it may be something else. What comes to mind is shotgun or artillery fungus which would most likely have been introduced on mulch. It leaves black spots on many different surfaces that are virtually impossible to remove. Here is some informaiton from Cornell University (with photos) and the Penn State University you may find helpful. You may need to cut and paste the complete url into your browser to make it work correctly.
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