What are the chances of a negative blood test in early pregnancy?

Very low. It depends on how "early" you are referring to, but generally, the blood test for pregnancy is very accurate with a very low false negative rate.
Can be false 17 days. A serum pregnancy test can be falsely negative up to 17 days after intercourse. Because urine pregnancy tests are less sensitive, it could take up to 3 weeks before turning positive after insemination.

Related Questions

I had sex June 10, I have had 2 withdrawal period since than and a negative pregnant blood test 28 days after sex. Chances of pregnancy still? On BC

Tests are accurate. Blood or urine pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A correctly done urine test gives a reliable result 3 weeks after sex (3 weeks after when conception might have happened) or 1 week after missing a menstrual period or a withdrawal period. Thus, pregnancy in this case is very unlikely. Read more...