Another thought, since I hadn't heard of Cocoa mulch at all before, are we sure it isn't Coco, as in coconut husks ground up? That seems much more likely as it's a commonly available garden product and would not be harmful to dogs.
What part of the Cocoa making process produces mulch? The husks of the fruit? Are we importing it from Brazil or Africa or what? It doesn't seem like it would be economically feasible.