Period is late! S.O.S

Hello! I am 6 days late on my period and I have been testing ever since, all of my tests have been negative but tomorrow I'm gonna do a blood test. Is it possible that the answer could change? I would like to know how accurate a test stick is to blood, I'm not ready to be a mother just yet. My boyfriend and i, we always use protection and he ejaculates outside but we're scared to death. All the comments positive please!