https://www.erowid.org/plants/cannabis/cannabis_testing.shtml

Blood Tests
Unlike urine tests, blood tests detect the active presence of THC in the bloodstream. In the case of smoked marijuana, THC peaks rapidly in the first few minutes after inhaling, often to levels above 100 ng/ml in blood plasma. It then declines quickly to single-digit levels within an hour. High THC levels are therefore a good indication that the subject has smoked marijuana recently. THC can remain at low but detectable levels of 1- 2 ng/ml for 8 hours or more without any measurable signs of impairment. Chronic users have been found to show residual blood THC levels of 1.5 (+/- 0.5) ng/ml for a full week after ceasing use [Karschner].

Blood metabolites
In addition to THC, blood tests can detect cannabinoid metabolites. Not uncommonly, labs report levels of THC-COOH, the same non-psychoactive metabolite found in urine. As shown in Fig. 5, THC-COOH levels for blood are similar to urine. They may be detectable for a couple of days after a single use or weeks in chronic users, and are therefore not a valid indicator of being under the influence. There is no scientific basis for treating drivers who have THC-COOH but not THC in their blood as being legally "under the influence."