Your stat about 50% of women having normal periods while pregnant is completely false.

*Some* women have spotting early in thier pregnancy. A *few* women have regular period like bleeding early in pregnancy. (it is very uncommon) However, no bleeding during pregnancy is considered *normal* and should be brought to your doctors attention.

On to the question. No blood will not effect test results. So if you think you are pregnant you should test. It will put your mind at ease.