When a presumptive test is positive, one should perform a confirmatory test. A positive forensic DNA test is not a confirmatory test for blood. "For example, while examining the clothing of a suspect, a forensic biologist might visually locate a brown stain that presumptively tested positive for blood and was then DNA typed. The DNA type is found to match the victim. Knowing that the loci tested are higher primate specific, what conclusions can be drawn? The only unqualified conclusion that can be offered is that the stain contains DNA that matches the victim. It has not been proven to be blood."
On the other hand if a separate
confirmatory test for blood came up positive, then one would conclude that blood was present. "The method of choice today is the ABAcard® HemaTrace test strips manufactured by Abacus Diagnostics, Inc....The pink dye becomes visible as a band in the test region at concentrations of human hemoglobin above about 0.05 µg/ml."