How to blacken the surface of stainless steel?

http://bbs.hcbbs.com/viewthread.php?tid=338479&highlight=%C1%D7%BB%AF%B4%A6%C0%ED This post mentions it; you can take a look

http://bbs.hcbbs.com/viewthread.php?tid=521386&highlight=%C1%D7%BB%AF%B4%A6%C0%ED Download of the book «Phosphating of Metals»

There are three methods for blackening stainless steel: the molten dichromate method, the alkaline low-temperature oxidation method, and the sulfidation method. The first two of these processes will be described below. I. Molten dichromate method: The stainless steel parts are first degreased and purified, then pickled in 8–12% sulfuric acid at 50–60°C. Following that, they are activated in a mixture of 1–2% hydrofluoric acid and 10–15% nitric acid for 1–2 minutes. Finally, it is immersed in a sodium dichromate molten salt solution and stirred thoroughly for 20–30 minutes to form a black film. Remove it, cool it, and then wash it with water. The temperature of the molten salt is between 450 and 550°C; below this temperature, sufficient oxidation cannot occur, and the black color cannot be achieved. II. Alkaline low-temperature oxidation method: The black film produced by this method is robust. Due to the low treatment temperature, a beautiful surface can be maintained. The treatment temperature ranges from 104 to 108°C, and the time required varies depending on the material, typically ranging from 10 to 30 minutes. The composition of the treatment solution is as follows: 15% sodium hydroxide, 3% sodium phosphate, 2% sodium nitrite, 2% lead peroxide, 0.1% iron hydroxide, with water making up the remainder. It should be noted that the alkaline low-temperature oxidation method relies primarily on the passivation film inherent in stainless steel; therefore, during preprocessing, after degreasing and cleaning the workpiece, there is no need for acid washing to activate it, as acid washing would dissolve the passivation film.

Hydrofluoric acid and nitric acid should be used; the exact formula isn’t clear. Read a book.