Does the STD herpes have to be present at the time to test positive?

No. Herpes carrier status continues to test positive irrespective of breakouts. Transmission can still occur without eruptions.
No. The blood test for hsv- herpes simplex virus measures antibodies- the bodies' response to an infection. So the actual virus does not have to be active on the skin to see antibodies -- there are different antibodies IgG and IgM - which can help with deciding how long the exposure has been.