The ones I tried did not do much of anything although these were cheap models. A regular black light bulb did better but only at night. The one thing they were useful for was finding arrows with florescent knocks, they worked great for this. Sometimes a red filter would make the blood appear white and stand out in the dark but it doesn't always work. Peoples eyes and especially color perception varies so it may work for other people.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli