As you've mentioned, the blood that you draw from the opponent and ingest allows a jashinist to create a connection between himself and whoever's blood he ingested. The sigil serves as the medium for the curse to be cast through. However, getting a drop of your opponent's blood is already hard enough, so you have your work cut out for you.
With that said, let's move on to the techniques. The technique below is very self-explanatory, so instead of performing it, I'll ask you to list the advantages and disadvantages of this technique,.
( Jujutsu: Shiji Hyōketsu ) - Curse Technique: Death Controlling Possessed Blood
Type: Offensive
Rank: Forbidden
Range: Short - Long
Chakra: 50 ( -20 per turn )
Damage: N/A ( dependent on jutsu used )
Description: In order to use this jutsu, the user must first create the Jashin insignia, a large 4 meter circle with a triangle on the inside, on the ground with his own blood. The user must remain in the center of the circle throughout the jutsu in order for the Curse to continue to work. After obtaining the opponent's blood in some way, the user ingests the blood and his face becomes black with various white markings that commonly resembles skeleton, a symbol of death. Once the sacrificial ritual has been done, the victim and the user are fatally linked. Any physical harm that the user takes will be reciprocated on the opponent, ranging from scratches to stabbing themselves through their chest.
Note: This jutsu lasts 4 turns and can only be used once every 4 turns.
Note: This jutsu can only be used by Jashinists.