Well (being a deep subject and all
) one sign of blood stains for me is when the area in question is pitted and blackened. Human blood is quite acidic and especially reacts with anything with iron in it (like steel). When blood is on a piece of steel for a while, it will not only etch but eat into the metal, oxidizing it black after a while. What may also help is if the area in question looks to have droplets forming a direction or spray. Since blood smears upon contact with a smooth surface, it will do so on a blade surface until it sits there and digs into the metal. I have a kampilan that has the blood spray in a direction where it cut. I have etched the piece to bring out the lamination and still cannot erase the blood stains because they have corroded deep into the blade, pitting the blade.