The requirements for taking a blood test for life insurance will vary by insurance company and what they test for may be different depending on the guidelines for the life insurance company.
However, most life insurance blood tests may check for illegal substances in your system, prescription drugs, elevated protein levels, HIV/AIDS, STDs, diabetes, marijuana, THC, methamphetamine, cocaine, and other illegal drugs.
If you tests positive for THC you may be declined for coverage; however, it may depend on where you live and if medicinal marijuana is legal in your state. If so, and you have a prescription for the marijuana, you may be accepted for life insurance coverage.
Here's an article that explains life insurance blood tests and how they work.