As Dfariswheel indicates, you'd need a FFL for such a business.
Once you need a FFL, you must be in compliance with all local and state zoning regulations. Here, you can often go down the rabbit hole, because blueing and metal finishing often comes under an industrial zone classification, and you'd need to situate your shop in an area zoned as 'industrial' or even 'heavy industrial' (depending on your community). If you don't get zoning approval, you don't get your FFL, end of story.
As far as EPA regs... Here's some light reading for you:
Personally, I don't need and don't want hot salt blueing in my shop. The only advantage of hot salt blueing is the speed with which steel can be blued. For all the hassle, regulatory compliance and personal injury potential, I'll stick with rust/fume/express blueing methods.