I assume you are talking about either the AFP test or the Nuchal Translucency Test when you talk about your blood test screening. As others have said, these tests only give you odds, not a definitive answer. If nothing unusal was seen on the ultrasound, odds are your baby is healthy.
HOWEVER - what exactly did the blood test screen positive for? Was it a neural tube defect (spina bifida, anencephaly) or a chromosomal one (Down Syndrome/T21, Turner Syndrome/T18, etc)? Neural Tube Defects can usually be seen clearly on ultrasound, whereas chromosomal defects can SOMETIMES be seen, but not always. For chromosomal defects, the only way to be sure is to have an amnio done. At 21 weeks, you should still be able to schedule one.
I would also ask, what exactly are they screening the baby's heart for? You need to know the answers to these questions before you can make an informed decision. Good luck and please let us know how it turns out.