Hello,
It is not likely the blood test was wrong, you could have very well gotten a false positive as well as it woulcd have been an evap line (depending on how dark it was) I know it is hard thinking you are pregnant but I would say no, the blood tests are extremely accurate.
Hope that helps
Personally I don't think a blood test would be wrong. If think it is you can always go to a pregnancy clinic to get a second blood test done.