Many commercially available black mulch products contain coal or other questionable ingredients to make them appear blacker. If your goal is to lay down black mulch for simply aesthetic reasons, then buy a deep, black-colored mulch and spread it on your garden. However, you can easily make your own black mulch that will be exceedingly good for your plants.
Lay down two sheets of cardboard and dampen them. Lay the cardboard directly in the garden where you wish to use the black mulch, or compost your mulch elsewhere and transport it later.
Spread 6 inches of steer or cow manure on top of your damp cardboard.
Spread 4 inches of grass cuttings, leaves and/or kitchen scraps on top of the manure. If adding kitchen scraps, do not add bones or meat, only vegetable matter.
Spread 2 inches of peat moss on top of the yard waste and kitchen scraps.
Add a second layer of all ingredients, starting with another 6 inches of manure. When your pile is complete, it should be approximately 24 inches high. Dampen it.
Throw a black tarp over the pile and allow it to "cook" for eight weeks. The pile may let off considerable heat at first, which is normal and good. Check the pile occasionally and spray with water if it shows signs of drying out. The pile should be damp but not soggy.
Mix all of the layers of the pile together with a pitchfork or shovel. You now have an exceedingly good black mulch fertilizer to spread on your garden.