The system for bloodtheft is really clunky and bogs down gameplay pretty easily. Here's the one I use.

Bloodtheft
You can perform bloodtheft by performing a coup de grace against a helpless scion. If the coup de grace kills the character, you increase your Bloodline score by the victim's Bloodline modifier (thus a character with a Bloodline score of 14 killing a character with a 16 would gain a +3 bonus to his Bloodline score). Thus, killing the Gorgon (Bld 60) would grant a character a +25 to their Bloodline score.

Corruption
Your bloodline can become corrupted by the overwhelming power of another character's bloodline. If your bloodtheft victim's Bloodline score is higher than yours, you must make a Bloodline check (DC 10 + 1/2 the victim's HD + the victim's Bld modifier) or immediately switch derivation to that character's bloodline. Thus, killing the Gorgon almost guarantees that a character switches to Azrai's bloodline (DC 10 + 20 + 25 = 55).

Becoming a Scion
A non-scion character can perform bloodtheft on a scion, as per the system for bloodtheft. Thus, a non-scion that performs bloodtheft on a scion with a Bloodline of 16 would gain a Bloodline score of 3, of the scion's derivation.