Is black mold on a bag of peat moss dangerous to breathe?  It is never a good thing to breathe black mold.

If you find this before making your purchase, leave it alone and go to another bag. If you have already purchased the bag and for some reason you then find the mold, my advice would be to wear a mask over your mouth and nose while opening the bag and using it. If the contents were questionable, I would empty them into an area where children and pets do not play. I probably would put it into my compost pile because it heats up and should decay the contents along with the other materials in my compost container. I still would wear a mask when working with this bag but it is not a bad idea to work with a mask when using peat moss at anytime.