well the idea is that
sign in blood says the person drawing the cards is the one who takes the damage. That's why when you get the emblem you want to use the sign in bloods on your opponent and the alter's
reap for yourself. That way all you have to do is sac a creature to get two cards and your opponent will draw two and take two damage (which would trigger the emblem effect and deal another two points of damage). So really drain and take as much as possible using his effect. Also one method that i've come up with is to use
vampiric rites to draw more cards, gain more health all for sacrificing a creature. If you did this with
youthful scholar you'd draw an extra card, if you did this with
black cat your opponent would lose a card, and if you played alter of
reap on your cat then your opponent would lose 2 life, 1 card and you'd gain 2 cards and better hand advantage. How does this sound to you?