According to Gabriel, the blood spell prevents the escape of beings bound to the caster, effectively preventing the Archangel to apportSam and Dean out of the Elysian Fields Hotel. It should be noted that when the archangel himself was bound, he had to order Dean to take away the blood of his vessel as well, indicating he could not simply teleport away. Also, seeing as he didn't simply leave his vessel and possess a new one, it appears that the spell bound Gabriel himself to her, not just his vessel. However its effectiveness over the Archangel seemed to be below absolute as he was still able to escape from (a short way) from Kali and leave a double in his place, all without her knowledge. The bind could only be broken once the caster herself lifts the spell, or if the blood used for the spell was taken away from the caster.
The spell required only the blood of the target, regardless of the manner of acquisition. The spell was not dependent on the amount of blood. As long as blood of any amount was acquired by the caster, the spell would take effect. The spell would also work even if another being collected the blood, as the case when Mercury collected the blood of Sam and Dean for Kali. Even beings who use vessels are not immune to the spell, as demonstrated when the archangel Gabriel himself was bound. The spell did not require any audible chant or incantation. It is unknown if this spell was unique to Kali or could be utilized by other deities or supernatural beings.