Why my boiling pot turned black?
You used a salt/acid mixture and, upon bringing it to a boil to clean a stainless-steel pot, parts turned black.
All stainless steel is not alike, and all are not “stainless” or non-reactive towards oxidation – given the circumstances.
Getting to your specific question:
The “black” you are seeing upon boiling is a Fe(II)/Fe(III) oxide, the same or similar to the black oxidative protection [the seasoning of a skillet] so valued by the fastidious owners of iron skillets.
One of many procedures for “seasoning” an iron skillet is similar to what you did.  I have done the following A) cleaning the skillet down to shinny metal, particularly important if you do not know the history of the skillet. B) rub a mixture of table salt dampened with an acid [vinegar] over the pan until you get the classic yellow-rust of iron we are so familiar with C) now add water to the skillet and bring it to a boil.  This step is critical.  The yellow rust will turn black upon boiling.  This is a protective Fe(II)/Fe(III) oxide coating.
Now repeat with the salt/vinegar rub and boil until you get a uniform blackish coating on the iron skillet.    Finally, fry up some bacon and eggs to give a carbon-based coating to the skillet.  The latter is more of a “paint” but with the black metal oxide coating as a base, your skillet is protected against “rust” and mild oxidation.
The “original Teflon ® coating” if you will.