The very first thing that must be done at all crime scenes is securing the scene. Law enforcement officials must limit access to the area/scene, this is done to maintain the integrity of the evidence that may be at the scene. You don't want scores of people trampling on the evidence.
If the evidence to be collected is blood, it must be determined by field test that the sample is in fact blood. This is accomplished by a simple test tube test using phenol derivatives. A small sample of the material is collected on a sterile cotton tipped applicator, it is then placed into a test tube with a known chemical reagent. A color change will occur if the substance is blood. This is called a confirmatory field test for the presence of blood.
Most blood samples at crime scenes are collected by this same procedure of swabbing with a sterile applicator then transported to the crime lab. Larger samples are collected differently. If blood is covereing a large area, say soaking a carpet, pieces of the carpet may be cut out by the crime scene technician and then placed in seperate bags for transport.
Chain of custody is important to all evidence collected at any crime scene. Essentially, this means that one individual affirms that the collected evidence never left his control until it was transferred to the crime laboratory.