Anything in the urine has to be in the blood to get into the urine. Therefore if thc can be detected in the urine for up to two months in heavy users, it can also be detected in the blood...... The actual blood level may help with timing to last dose, but the simple presence or absence of the substance does not
You’re ignoring the information given that says THC cannot be detected, by tests, in the blood more than two days after consumption. I hate to break it to you but the kidneys filter the blood. The metabolites that the kidneys filter out are concentrated in the urine. Obviously in higher levels than the blood itself carries. There must of course be THC in the blood for the kidneys to filter out but it doesn’t have to be in concentrations high enough to detect. Otherwise they wouldn’t say that it can’t be.