The blood test does not detect the virus. It detects anti-bodies that your body produces against the virus.
For the majority of people the most accurate indicator is 12 weeks post exposure. It is possible to be infected and still have a negative IgG result as your body hasn't made enough antibodies yet at the time you took the test. Yes, it is possible if your partner is positive and you've had a herpes outbreak, to still test negative blood-wise for the same reason, your IgG haven't been produced in mass amounts yet.