If a blood test says you're not pregnant can you trust it?

Yes: the tests are very reliable. You could theoretically have a false negative, but it's uncommon. If you test too early, say less than 10 days after ovulation, you'd expect to get a (false) negative as well. Hope this helps.

Yes absolutely: it's the features around the test that can be messed up. 1 day post conception won't show up on a test. 10 days or more post-conception will..later is more reliable. People get tested at the wrong times.