I dated a girl with herpes for over a year and was never infected. The test isn't entirely useless, but mostly. Anyone who has ever had a cold sore will come up positive on the blood test, the only way to know for sure is for her to have an outbreak and have a doctor confirm it then. Avoiding infection isn't all that hard, and definitely worth it if you care. My partner told me on the second date as we were about to get hot and heavy. We waited a couple of days for me to figure it out, but in the end it was worth the risk. Which again isn't all that hard to mitigate. She took control medication, we ALWAYS used condoms, and when she did have an outbreak (which are usually stress related, so you can sort of predict based on how life in general is going) we wouldn't have any genital contact. I got lots of blowjobs then
we would wait a couple of days after everything had cleared up just to be sure. There are also often some mild kinda flu like symptoms (achy and lethargic) that often accompany the onset of an outbreak so she would know it was starting. It really was kinda like a random period that would show up every 2-3 months for a couple of days. Being open, honest, and non judgmental is the way to keep these things under control. There are definitely plenty of people who have it and are afraid to talk about because of the stigma, some know and some don't, and many go on to infect others with or without knowing because they were afraid to have the discussion.