Was your blood test a quantitative or qualitative? In other words, did they give you a number, or just a yes/no? A couple of things, the yes/no blood test may be a "no" if done very early in pregnancy, before you have enough hcg in your system to register on the test. The quantitative test, where they give you a number tells how much hcg is in your system, less than 5 is not pregnant. The other possibility is that you had some hcg in your system, because the egg was fertilized but didn't implant properly, sometimes called a chemical pregnancy. If you continue to have + tests at home, you should get another blood test to see if your hcg is increasing.