The Shulhan Aruch (318:9) rules that a Pot retains its status as a Keli Rishon, even after it is removed from the fire, as long as its contents remain hot-Yad Soledet. Therefore, in general, it is prohibited to put raw food into such a pot.
Nevertheless, the Gemara records an opinion that salt is an exception to this rule. Since it is very difficult to cook salt, it is only a violation when put in a Keli Rishon on the fire. Maran (318:9) accepts this opinion and allows pouring raw salt onto the contents of a Keli Rishon off the fire.
However, there is another version of that Gemara which holds the exact opposite: Salt is extra-sensitive to heat and becomes cooked even in a Keli Sheni. The Rema rules in accordance with this stricter version.
Sepharadim may follow Maran and pour salt directly into a Keli Rishon off the fire. However, it is prohibited to pour salt onto the contents of a Keli Rishon on the Blech or hotplate.
This discussion pertains to the raw salt mined from the earth. It is possible that the process used today to extract salt involves cooking. If so, it would conceivably be permitted to pour such salt on food on the Blech or hotplate. Nevertheless, because it is not clear exactly how each type of salt is processed, on should avoid doing so. This is the opinion of Hacham Ovadia and Hacham David in Halacha Berura.
One should only pour salt onto the contents of a Keli Rishon after it has been removed from the Blech or hotplate.