Lots of different processes for blacking/blueing or browning steel. Some – for example steam heating, or boiling in strong caustic soda solution – are mostly harmless. Others use chemicals covered by the Poisons Act or are licensed explosive precursors. Take your pick: A molten mix of Sodium Nitrite and Sodium Nitrate which fumes toxic Nitrogen Oxides, or wet mixes containing various combinations of Mercuric Chloride, Antimony Chloride, Bismuth Chloride, Selenium compounds, with Phosphoric, Oxalic, Hydrochloric and/or Nitric Acids. Now spoilsports have banned simply tipping toxic waste into the nearest river at zero cost, disposal of effluent is expensive.
Bloody tree-huggers, there's no proof Mercury in drinking water ever harmed anybody…
As always in engineering, production is a series of balanced compromises with cost playing the major part. Methods using heat alone are cheap and safe but the protective film is thin and the metal can be damaged internally. Chemical methods produce thicker protection and lower temperatures but are expensive and likely to involve unpleasant chemicals requiring special handling. The good safe cold processes don't always fit the bill. When all the costs are added up a particular blackening process could be more trouble than it's worth, and dodging the bullet by switching to stainless steel could well be advantageous. But blackening is still available and appropriate in the right circumstances.
Dave