LOL!!
That little detour to the 'Spear of Longinus' was pretty good Atlantia! and its pretty amazing how instinctively everyone responded to what many would consider an extremely esoteric topic.
The subject of blood on old weapons, is of course a much romanticized notion. Often in conversations with people not involved in antique weapons, when I mentioned collecting swords, would curiously ask if any had blood on them. Since the unpleasant reality of weapons is thier true purpose, it seemed a logical question.
Since I typically went for weapons which were in stages nearly at the excavated level, the dark corrosion and deep patination made any such estimation unlikely. However, on some blades there would be patches of dark, blackened corrosion, very much like the black blotches described by Mark Eley . I often wondered if internal degradation or impurities might account for these in some way. I have understood that often the corrosion of iron objects can develop internally. I had an old cannon ball that retained its external integrity for many years, then when moved, literally disintegrated.
Since most weapons would have been cleaned after combat, it would seem that only weapons lost on the field of battle would be potentially viable for this phenomenon, and even then, it would be difficult to identify the corrosion from blood from other degradation over time deposited in the elements.
In many African and some other cultures, a weapon which had killed was considered tainted and had to undergo considerable cleansing and other ceremony to avoid the wrath and revenge of its victims. Even in medieval Europe, it was thought in early medicine that the anointment of the offending weapon was essential to heal the victim wounded by it ( uncertain of what was done if the victim was killed).
Turning to sacrificial weapons and objects, an interesting article from the New York Times ("Ceremonial Objects from West Africa with Blood in the Patina", Henry Fountain, December 2,2007) notes that wood sculptures in animal and human shapes known for dark patina have recently been revealed to have traces of blood potentially explaining the coloration. The eight items of Dogon and Bambara origin from Mali were studied by French scientists using precision chemical analysis and some of them contained heme (the iron containing molecule bound to hemoglobin). Some of them had only iron left from the proteins, while the heme had degraded away. These were late 19th century to early 20th century, though one described as over 500 years old.
While the scientific complexity of this study is far beyond my understanding, it is noted that such study is indeed possible for objects of wood, and I wonder if similar processes could be used on metal.
The discoloration of the patination also reminded me of a most interesting movie I once saw, "The Red Violin", which was themed along with the secret of the Stradivarius instruments, and suggesting that another maker had actually used blood in his varnish to captivate its ethereal sound.
Just some more thoughts on this interesting topic.
Addendum:
Just found another New York Times article,
"Stains on Prehistoric Tool are Human Blood" John Noble Wilford 12/1/87
Dark brown stains on 100,000 yr old stone cutting tool recently tested by scientist at U.C.Calif at san Francisco using radioimmunoassay have proven the stains are of human blood. These artifacts found in 1950's at what is known as Barda Balka site in Iraq were cited as having blood stains early in 1980's but proved controversial, though now proven by the new research.
In another item concerning blood stains on old weapons, noting the interest in earlier forensics :
"...when of old standing, or a rusty piece of metal, it is a matter of some difficulty to distinguish them from stains produced by rust or other causes".
"Medical Jurisprudence"
A.S.Taylor & E.Hartshorne (1856)
While obviously dated material, this simply illustrates that literary romanticism and ambitious arms enthusiasts comments in Victorian times clearly were issued independantly from scientific positions of the period.