It's an iron oxide that forms during the forging process and is food safe just make sure you wash it before use
Also the iron oxide is good because it protects the steel from corrosion (but be careful about the shiny part as it will still corrode if not treated properly)
The iron oxide layer also just looks good because it says, "Yeah this knife was hand forged!!!" or it can give the blade a post apocalyptic vibe to it