There is nothing in normal B&W chemicals that will be combustible to my knowledge. My concern is for your safety. Being a darkroom, you will be in a small and enclosed (and sealed) area. Unless you have a forced ventilation, you may actually suffer from lack of oxygen and if that should ever happen, you will not be the one to be able to tell it. Plus, incomplete combustion, if that should happen will leave you with carbon monoxide which is odorless. Pilot light is always on but full-on burn will come on from time to time and that will consume good amount of oxygen. Maybe chemists among APUG membership can give you a quantitative answer but that will be my concern.