Caravanman, steam IS invisible...provided it's not cold out! That's why the most dramatic steam photos are taken in the winter...the cold air condenses the steam into a great visible cloud!
As far as the black smoke...speaking as a professional boiler operator, that's just poor combustion. "Modern" (late 1920s and after) locomotive boilers are usually equipped with overfire jets which introduce additional combustion air above the fuel bed and, when properly adjusted, should eliminate most of that. Black smoke is just wasted fuel. But sometimes, especially for a photo runby, you get a fireman deliberately "making a show" by burning excess fuel with insufficient air, or perhaps "sanding the flues" ...a technique to clean soot (the sand blasts it off) while at the same time making a big plume of black smoke.