A blood test is designed to detect the presence of marijuana in the blood.  Unlike urine tests, blood tests do not detect metabolites.  Usually, for this reason, blood tests are only performed immediately after an accident to determine if the subject was impaired at the time of the mishap. Blood tests are also very costly and are usually only requested by insurance companies.

You most likely will never encounter a blood test as a routine pre-employment drug test. Instead, urine drug tests and saliva tests are more commonly required for employment purposes.