5-8 days late in my period. two neg tests. very reg cycle usually. could they be false neg? should i go in for a blood test?

Not false negatives: Any test can have a false negative result (that's life), so the question is not can the result be a false negative, but how likely is it a false negative. In this case, 2 urine pregnancy tests, done correctly, 1 week after missing a regular period... Means one is not pregnant. If worried, one can try another brand of urine pregnancy test in a week. Or, just wait to see if the next period comes.