I discovered the is no way to wash the blood off maces, bludgeons and clubs.
Steps to reproduce
Simply make you mace, bludgeon and club bloody (hit any animal or human).
Bugged behavior
Now you have an unwashable bloody mace.
Why this is a bug
Trying to wash yourself in a trough doesn’t do the trick of washing the blood off, same is for visiting the bathhouse. Naturally, you are also not allowed to use a grinning stone to “whet” maces, bludgeoned and clubs and get rid of blood on them that way.
So, as far as I can see, there is no obvious in-game way to wash the blood off a mace.
Fix suggestion
Maybe you should tweak washing in the baths to make it also clean off blood from active weapon (the one in right-hand slot). And, also, do the same thing for washing in troughs.
Game version
1.9.0, PC, GOG-version