We would only suggest using black sheets under two conditions: 1. You can obtain them relatively inexpensively. They can be costly, and you might need many. 2. Most importantly, no matter the type of haunt you're doing - home, charity, or commercial - the safety of your guests is paramount. If you use sheets, you must spray them with something that is flame retardant that has the approval of your local and state fire laws. In our area, we used black plastic sheeting with a high flashpoint for such a material to cover our original pallet structure, and then we had a flame test done by the local fire department.
Most materials you're going to be using for construction will burn. You want to look for things that give you and your crew time to react and handle a situation, should one arise. Make sure you have plenty of fire extinguishers on hand, your actors know where the closest one is to their area and they know how to use them as well. If you cover those two basics; fire retardant/high flashpoint and readily available extinguishers, you can use any darkening material.