The clinic schedules a blood test when it does because that is the first day it will be reliable overall. An embryo can float around for ten days or more before settling in and picking a spot. As long as it is still floating, it is not creating an hCG signal. A negative test at 8 days doesn't necessarily mean negative, though I would believe a negative test at 14.
Thanks for the info. Trying to stay positive, I'm just not ready to give up hope yet!