The reason for the blood test is to determine if you have any health issues which may result in your life expectancy being shortened - meaning you will die sooner than normal based on the health issues shown in your blood test.
The blood tests may check for health problems including diabetes, high cholesterol, high protein levels, STD's, HIV/AIDS, tobacco use, and use of legal and/or illegal drugs; such as, cocaine, marijuana, methamphetamine, and heroin, among others.
The results of your blood test may change your rate for life insurance or cause you to be turned down for a life insurance policy. You may choose to dispute the blood test results and request a new blood test, but that is up to the insurance company.
Here's how you may qualify to buy life insurance without any blood test required.