Bloodmark can be removed by using an Altar of Cleansing found in town.
The Altar of Cleansing is a Point of Interest location found within the two Fields of Hatred. They are used to remove your own Bloodmark once you have activated your Mark for Blood.
The Altar of Cleansing in Diablo 4 is a PvP feature that can be interacted with within the Fields of Hatred in order to remove a Bloodmark. The Mark for Blood in the Fields of Hatred allows players to attack others and be attacked by other players within the designated PvP zones. Players can only activate this mark within the Fields of Hatred. There are two Fields of Hatred locations on the map. Players who do not wish to engage in PvP are not required to display a Bloodmark. Players without a Bloodmark can still enter the PvP zones and engage with monsters.
If you wish to end your PvP interactions players can then interact with the Altar of Cleansing to remove the Bloodmark. Players can still remain in the Fields of Hatred and PvP zones however they will not be able to attack other players or be attacked by other players.