The use of blood in rituals and spell work is as old as magic itself. Using blood is a way of powering your magic with a bit of your own life-force, and that can be a very powerful thing. However, if blood is to be used it not be blood from an animal or from someone who did not consent to its use. In fact the whole point of using blood is that it is your own blood giving power to the ritual.
There are times when using a bit of your own blood might be a very appropriate component to a magical working. If you are doing a working which involves the concept of personal sacrifice what more powerful sacrifice could there be then a bit of your very own life force?
Or it may be something which would be an appropriate sacrifice for the God or Goddess with whom you are working. Working with a deity associated with warfare and battle may mean that blood is the most appropriate offering for their assistance. My personal Patron is Sekhmet for whom blood is a traditional sacrifice. There are other sacrifices which can also be used when appealing for her aid such as red beer or water. However, if I am conducting a ritual where I really need and desire her assistance, then a few drops of my own blood are not only appropriate but is the sacrifice most apt to get her attention. For instance, when my mother had open heart surgery last summer it was some of my own blood I offered to Sekhmet to aid in a working to bring my mother through her surgery successfully.
Traditionally blood may be used to empower certain tools. In Norse practice the runes were and are traditionally empowered by adding a drop or two of the user's blood. This is a way of making an offering to Odin for the sacrifice he made hanging on the World Tree to gain the runes. And there the act of using blood on the runes ties them closely to the user.
So the upshot is that it is truly up to the individual as to whether they wish to use blood in their workings. If you do not wish to use it, then don't use it. But there are good reasons for the use of blood, so please don't write the concept off as being somehow wrong or dangerous and the people who do use it as being unethical.