Hi, I´ve seen this on another server and i thought it was a really cool and small addon. All it was, were redstone particle effects coming off of a player when (s)he got hit. If i'm not mistaken, it's really easy to make, i just don't know java language. Thank you to any developer that is willing to try attempt making this!
From what i'm aware of there is already a public plugin just like this, that is up for download, but i seem to forget the name.
I have made you the plugin. http://dev.bukkit.org/bukkit-plugins/playerblood/ It is my first plugin, so let me know if you like it.