The bloody mourner top is an item received during the Mourning's End Part I quest after ambushing a Mourner on the Arandar pass. Players must clean this using Tegid's soap, obtained by stealing from Tegid's basket, before they can wear it.
Use the soap with the Mourners top while having a bucket of water in your inventory. It will be cleaned and you will be able to wear it. After a player receives all of the Mourner gear it can be stored in the Costume room of a players house.
For an exhaustive list of all known sources for this item, see here