OK, my curiosity got the better of me and I had to know what a black light shows when dealing with blood, old or new. The answer, nothing! I purchase a rather high end black light with directional bean some time ago, no not for collecting, but to see if our puppy at the time was piddling on the carpet! The vet warned us that Cocker Spaniel females will piddle when excited and he was right!
The minute urine areas would glow very brightly under the beam, it really helps with spot cleaning the carpets when needed. So I though OK that is a bodily fluid, so will this work on fresh and old blood, resounding "no".
I like to experiment and test a different hypothesis from time to time, so I took one for the team and cut my finger, smeared it on a repro forage cap and voila, nothing! I also shined the beam on the interior of a wheel cap in my collection that belonged to a trooper in the 1st VA Cavalry Co B and worn at Manassas. The interior of the cap has what appears to be obvious blood staining, not from a wound, but most likely a small cut, mosquito bite or plain scalp rash due to the Virginia heat.
So back to luminal, though i do not recommend it unless you have to know, urine apparently shows up due to the high ammonia content.......
The cap is in marvelous condition and very likely put away as soon as the trooper could acquire something a bit cooler, see below:
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