When gas molecules are not burned completely by your home's propane fireplace, those molecules are released into the air and collect on surfaces as black soot. You normally don't see the molecules escaping from the fireplace, but you will see the gas molecules when they collect on a surface. Black soot stains any surface it lands on including walls, fabrics, flooring and furniture.
Burning propane gas produces water vapor and carbon dioxide molecules that are both colorless. If your fireplace's burners and pilots are dirty, the propane gas will not burn properly. Molecules will then escape into the air and form black soot on surfaces. Over time, your propane fireplace's components also become worn and unburned gas molecules escape from the components and travel through the air in your home. The molecules clump together as black soot and can cause respiratory problems if it's not removed.
Using incorrect propane gas components or orifices on your fireplace can cause propane gas to not burn properly resulting in black soot. This issue usually occurs on natural gas to propane gas conversions. Black soot can also result from a malfunctioning propane gas regulator. The faulty regulator may be letting too little or too much gas into the fireplace causing a percentage of the fuel to not burn properly. Also, making incorrect pressure adjustments can result in the gas regulator malfunctioning and gas escaping from the component.
Using your propane fireplace in a room without enough oxygen or that is too small can also cause the gas to not burn properly and escape into the air. You will notice black soot in the room with the fireplace as well as in rooms next to it.
Have your propane fireplace serviced by a qualified technician to resolve the black soot issue. Propane gas is dangerous and adjusting the fireplace's components yourself can damage the parts and put your home at risk for an explosion. Try not to use the fireplace until it is checked and serviced. Open a window or door in the room or area containing the propane fireplace so air can flow into the space if you have to use the fireplace before the technician can visit.
If you see visible black smoke escaping from the fireplace, turn the propane gas regulator to the "Off" position immediately and call a technician -- your propane fireplace is giving off a mixture of gases including carbon monoxide which is extremely dangerous.