Blood tests really don't tell you yes or no. They give you quantified biomarker concentrations which are correlated with certain types of cancer. In some cases the diagnosis is clear, but often it's more of a gray area.
True, if the level found is 150% of normal, it's a problem. In my case, I was misdiagnosed for 10 years. When my docters finally ran the test for the right biomarkers, they found levels 50 times normal. So maybe the tests need wider margins to screen out false positives.