Herpes can be detected with a blood test (http://www.ashastd.org/herpes/herpes_learn_testing.cfm#3), though blood tests can only tell you that the HSV antibodies are present in one's system -- not that a particular lesion is or is not caused by a strain of HSV. If the blood screen mentioned in the OP included herpes (which is not terribly likely -- in the absence of symptoms, HSV is not part of most STI screenings) and if the blood test returned negative results for HSV-1 and HSV-2, that would pretty much rule out HSV as a cause of this particular sore.