Negative blood?

Tay

Has anyone experienced a negative blood test and ended up still being pregnant? I know it's normally very accurate with blood, but I know some people never show HCG. I was supposed to start my period on the 19th, and nothing yet. I've never been this late before. My boobs are so sore, and I feel nauseous, but idk if it's me just being crazy or what. I had a blood test done Friday and it was negative!