In Ethics and the Craft, John Coughlin briefly discusses blood magic and why people don't use blood with magic. The original reason for not using blood with magic is that the energy released isn't stable or reliable. I've found a similar claim made in Stephen Mace's book Nemesis and other Essays, where he discusses how one of the Roman emperors abused blood magic, to the point that it caused him to make rash decisions.
Blood is representative of a person's life force. We can't live without blood and so it is a powerful agent of transformation that can be used in magical work. I've used my own blood in my magical work. For example, when I made my pact with the elementals I gave them my blood and I've done similar pacts with other entities and shed blood as part of the pact. In such a case, it is an exchange of my life essence for their life essence.
I've never drawn on my blood for any other magical working, but I also know of cases where people who have chosen to donate their blood have also chosen to enhance its healing capacities with magic. Certainly a worthwhile way to utilize blood magic.
I respect blood as a fluid of life and as a symbol of life energy. I wouldn't write off drawing on it, though I think the only blood you should use is your own. I do think its important to be careful with it. Use sterilized tools if you are going to draw blood and have respect for what you are drawing on via the blood, i.e. your life force. I suggest using Lancets, though I've also used a knife in the past. Lancets are much less messy and you can get a few drops of blood from a finger and have it heal fairly quick.
Blood magic isn't something to do lightly. As I mentioned, I only use it for making pacts with spiritual entities I want to work with and I don't even use it for every kind of entity I work with. There are specific circumstances that warrant it, such as when I wanted to develop a closer connection to the elemental spirits. I reserve it for those occasions and those occasions alone.