A lot of us just do whatever the situation calls for at that time. Black or white often places a moral conotation and not everybody shares the same morals and values. Color is a good personal definition but because it is personal to each person, we don't all share the same definition on it.
I have been searching a lot about the division between black magick and white magick too. I think that the intention is a good way to make the difference, but I have learned that even the ingredients have a big value in it. For example, using blood sprinkles to make herbs and plants to grow faster, it's black magick.
Bottom line is that magic is a tool, like a hammer. it is a collection of ideas and practices that gives a person materials to work from to commit will and intent towards a specific goal.
A hammer might be a vital part in building a home, but it can also tear one down. or crack a skull. But it is still just a hammer. *laughs* i find it fun to compare this particular topic to the riddle of steel from the original Conan movie (IE; the good one.) The power of steel does not come from its shape or construction, but from the hand that wields it. It is not the knife that harms or heals, but how its function to cut is applied.
When talking about the use of individual tools, and whether they might be black or white/left hand or right hand/additive or subtractive, it is a matter of how they are used and why. Context plays a huge factor.
Using blood as an example;
As a medium, blood is a direct link to your energy and being as a person. Physically blood carries the nutrients and oxygen that you run on, it is also a carrier of your defenses (immune system) and DNA. Philosophically Blood is referred to to represent one's family and lineage, conencting to your family line, ancestry, and future generations. Spiritually, the flow of your blood paralells the movement of your energies. So, quite literally, your blood is -you- expressed physically spiritually, and mentally/emotionally.
When using your own blood in magic it is an act of forming a direct link between yourself and the intended working. Think of it as an act of dedication towards the goal of the working. This is also how it expresses itself if labelled as a sacrifice, as you are giving up a portion of your essence and making it a direct part of the energy of the magic.
As far as using the blood of someone/thing other than yourself, that is where it can become sticky, and requiring the answers to some very important questions; is the act voluntary? Is it an act that will cause lasting harm? is the subject human or animal? What is the purpose or goal of the working?