It's helpful if you give us your specific scenario - did you test for herpes, and if so for any specific reason?
A blood test is very reliable, but it depends on certain factors like timing and the actual numeric results. A culture as well is completely reliable in a true positive way; but as Tired said it also carries a high false negative rate as well if taken too late.
Well if you get a positive culture then you can be sure that you have herpes and you know the location. BUT cultures have a high false neg if you do it past 48 hours!