While it is possible that the blood test could miss your infection (it misses 1-2 of 10 infections with HSV 1), it is far less likely that it would miss an HSV 2 infection, which is what I think you are most concerned about. The majority of the people in the US have HSV 1 infection already and if you finally did test positive for HSV 1 by a better antibody test, you could not know if the infection was oral or genital, only that you had it.
There aren’t too many things that cause water blisters in the genital area other than herpes but there are a few – some kind of allergic reaction, a staph infection might cause what you would consider to be clear blisters. But without a swab of the lesions from three years ago, there is just no way to know for sure.
Terri