Where can I buy black mulch in bulk in southern Maryland or Montgomery County, Md. I can pick up with truck or have it delivered. Thanks
Check to see if you have any mushroom growers around.
They sell their used mushroom compost fairly inexpensively, and it is usually black.
Some people in my area get theirs from mushroom growers near Philly.
Another growing source is local cities and towns that have their own composting systems. Some of them will sell it, especially to residents. You might want to check your own jurisdiction.