Blood Brothers / Blood Sisters is a form of unity that can be forged between two of the same gender aka close friendships.
Normally, if a male and a female wanted unity they'd have a marriage, for the longest time this was the only thing allowed, Blood Brothers (BB) / Blood Sisters (BS) didn't exist in the earlier versions of NexusTK but later came to pass. People choose to do this as a form of Role-play or just for someone to Hunt with. The BB/BS receive a special stone called a 'Blood Stone' which allows the user to teleport instantly to their sibling's location (it is much like the Love ring) with a few minor restrictions. (You cannot teleport to the location of your sibling if you do not meet the level requirements for the cave they are in, and you must be in the same area/server)
To forge a Blood Oath you and the prospected sibling should travel to Blood's House (KaMing's Encampment 12,46) and say 'Blood Oath' follow the steps, and there you go - you will both receive a Blood Stone and a Legend Mark that says " .... Blood Brother " or " .... Blood Sister"