There is no problem with the PCR swab test for varicella zoster virus and since there were also lesion on your penis, that is a test that we would have done on this rash but clinicians vary in their practice. And it does sound like this was a pretty classic shingles presentation though you are on the young end for this infection. The top of the penis is on the midline, yes. Hard to know what to make of that alone, but that isn’t the only thing going on here.
The herpes blood testing does have a problem with test results in the low positive range. But we can almost always clarify that with another confirmatory test. But again in this case, that may not be necessary, given the clinical presentation of pretty classic shingles.