If someone ovulates and concieves and then a week later takes a pregnancy blood test, how accurate will that be?

Not if it's negative: A positive test is pretty accurate, but a negative test is not reliable that early. It can take 11 days after conception to show a positive blood test result. To make it more complicated, since sperm can live for up to 5 days inside a woman, and ovulation is sometimes difficult to pinpoint exactly, she can conceive later than she thought, and need to wait longer for a positive result.