It isn't that blood magick is stronger or more dangerous, it is that blood magick is more binding. Think "signing in blood" vs. signing with a regular ink pen. Which one is the attention getter? Blood is life, without it we die. An oath in blood is an oath to be held high. Blood is also specifically circulated by the heart, something we relate to emotion. So to use blood is to have one's heart in the magick working, a little literally! You are putting yourself into the working not only emotionally, mentally, and spiritually, but physically too. That's some serious stuff. That's why it's necessary to really be certain that you want what you're aiming for, your ritual is completely sound, tried and true, and you are a mature and experienced magick practitioner.
Take care when using blood. And do not use blood magick along with masochistic tendencies. It is meant to be demonstration of passion for the goal and not "enjoyed". Only a single drop is needed. We have written extensively about it on this board, so use the search function above to search "blood magick" and "blood magic".
Also Google "Socarie Blood Ties" for a very nice collection of information on the subject.