People do not seem to understand what blood magic is. And frankly, the constant repetition of it is rather annoying. Okay, what Supernatural episode did I miss that has everyone obsessed over blood?
Please use the search bar because I've posted on nearly every one of these "blood" related topics.
Blood magick is using a part of yourself in magick. It is giving one of the most intimate parts of yourself that you possibly can, the very fluid that keeps you alive. While semen is considered a man's strength and power, and menstral blood a woman's strength and power, we are still alive without these things. Blood magick is an itty bitty sacrifice. We are not killing anything, but we are offering a piece of ourselves, a piece of something very vital. Magick is all about the symbolism, and blood is symbolic of life and spirit, some may even say the soul.
Why would you attach yourself to a force you are using to push something away from you?
It would really all depend on what you were "offering" your blood to. You wouldn't just slap blood on something. Blood is precious. I use blood as a way to "seal" vows to show my dedication, and as a way to stain rune carvings because it is a way to feed the force of the runes themselves and bring them to life. Other than that, I rarely use my blood. I've used menstral blood far more often, not that you probably wanted to know that. ;p
The largest danger with blood magick is infection. It was back then and it is now. The other is having something like hemophilia (the disease that can cause you to die of blood loss from a paper cut). You need to be very careful and one drop is all that is needed. It's also good to take care of cleaning up after yourself well. Blood can carry contagious diseases and we don't test ourselves for those everyday.
The second danger is the spiritual one. Rule of thumb? Never offer your blood to a spirit unless it is a deity you know and trust. Would you give your blood to a random witch on the street you didn't know? I'm sure you are aware of how we use personal possessions in ritual. A strong spirit you are evoking (demon, angel, djinn, sidhe, whatever) that can manifest change in this world shouldn't have a symbolic source of your life given to them because you thought it would help get whatever you wanted. This is why people come here later on crying "help me, I'm being attacked by a demon". They aren't going to admit to being incredibly stupid. Honestly, it's hard coming back from that mistake. So just don't do it.