Blood tests are 100% accurate at detecting the presence of hcg in your system, which generally indicates pregnancy but can be due to taking a medication containing hcg. They are generally done at or around the date of your last period just like a home pregnancy test. The difference in the blood test is it detects an actual level of hcg, which can be lower than that detected by a home test. Really, that only gives you a few days before the missed period for most people since home tests now detect hcg levels as low as 20 mIU/ml.
Typically, no (from my personal experience). I have, however, heard of women who do have light periods throughout pregnancy.....including a friend's daughter who DIDN'T KNOW she was pregnant the WHOLE WAY THROUGH!!! Sounds crazy....but it can happen, I guess.