black salt
/ˌblak ˈsɔːlt/ /ˌblak ˈsɒlt/noun
Any of various inorganic salts that are naturally dark in colour, or are darkened by admixed charcoal, soot, etc.; specifically (in plural) impure potash obtained by leaching wood ash and evaporating the resulting solution (now historical ).
Origin
Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator. From black + salt, originally after post-classical Latin sal niger.