The herpes swab is looking for the herpes virus in a particular area of your body. It is a very sensitive swab and can detect herpes, can tell what type of herpes and also what part of your body the herpes is on so it is a very useful test for information that you can use in the future. The blood test is not done routinely because most people have a positive result having been exposed to herpes through oral contact from childhood, parents or siblings with coldsores, sharing drinks/straws, playing rugby/ wrestling, kissing etc. The blood test does not tell you where on your body the herpes virus is and the initial blood test will not tell you what type of herpes. It is quite possible to have a positive blood test that shows  herpes but have a negative herpes swab from an area of the body that was swabbed as that area does not have herpes.