to add onto what Hollabaluru said, while it is a fact spells are stronger if the one who is casting is the one who needs the spell to succeed, adding personal objects, as well as parts of oneself [hair, nails, blood, spit] can increase a spells energy greatly as well. while not everyone casts this way, it is a choice, blood magick incorporates a persons blood into a spell in some way to personalize it and give it more energy. this blood is typically the casters, but if the spell is being cast over another, you can use theirs as well.
blood is a life force, so it carries with it a lot of energy, you can add it to any spell, even if it doesn't call for blood. i've been taught you prick a finger on your power hand, but it can be taken from anywhere i think. i also heard menstruation blood holds a lot of energy, but i've never cast any blood magick so i can't give you a yes or no.
some deities [like Sekhmet] favour blood, and while you can substitute beer dyed red as an offering, it is best to leave out some blood. it could be cows blood [can be acquired at a butcher] but for life changing things offer up your own blood.
blood magick, like all magick, isn't good or evil, it doesn't work for everyone, and isn't something one needs to cast. research it and decide for yourself. if you think it would give your spells a personal touch as well as more strength but blood grosses you out, or you pass out when cut/injured [like me, and no it's not the site of blood] but you still wish to incorporate it into your casting, go with spit or another body part/fluid. blood is just seen as the most powerful. as long as you charge with energy though you shouldn't worry.