I found some information, though it does not specify what type of acid was used. It seems that this etching work was done primarily in the Mahdist arsenals at Omdurman.
The weapon was blue heat tempered or fire blued until the surface forms a thin layer of magnetite, upon which the blued surface has the design or in this case inscription , applied with a mastic resin resist material , then the acid is applied. The acid pits the exposed metal leaving a raised design where the resist was applied.
When the kaskara (in this case) or weapon was finished, it was bright with dark blue etched writing. This must have been an incredibly dramatic appearance which would have deeply inspired the ansar who would wield these weapons. Naturally over time and wear, the inscription would become metallic as the bluing wore away, while the background of the blade would darken in contrast.
This is my obviously lay interpretation of this process, and I would beg the indulgence of the metallurgists reading this
but I wanted to at least give what perspective I could to how this was done.