Blood's composition consists of 90% water, meaning that a capable waterbender with the knowledge of the body can bend the water situated in the blood, therefore resulting in full control over a person's body. Given the technicality and preciseness needed to perform bloodbending with sophistication and control, waterbenders only use the technique under the full moon, wherein waterbending is at its peak.
There are, however, extremities associated with the use of bloodbending. In truth, a waterbender should be able to freeze or heat the blood within a given person's body, instantly killing them. This, of course, would be much too extreme for a children's TV show. In addition, the idea of bloodbending itself is immoral- reaching into another person's body and enforcing your own will onto theirs. This is why Katara rarely uses the technique, given the fact that she believes it's a dark form.