blackening solution
A chemical solution used to blacken metal surfaces, often in industrial or craft contexts.
We applied a blackening solution to the iron surface to perform a black rust treatment.
developing solution
Archaic term for a photographic developing solution; modern Japanese uses 現像液.
See also: 現像液
In old photographic techniques, it is recorded that a developing solution called 黒化液 was used.
Compound of 黒化 (blackening) and 液 (liquid, solution). The term is technical and its historical derivation is straightforward from its components.