Well, a relation is something shared, so, that would be like brother and sister from the same mom and dad. If you are in a 'blood relation' with a boy friend or girl friend, then that is mostly known as imbred or cross breed. So, a blood relation is a relationship that is cousin or brother or sister that are in any kind of relationship.
Blood relationship is a close connection amongst family members that share common ancestors. People within a blood relationship can also share some common traits such as hair color, eye color as well as the color of the skin. You have a blood relationship with your parents, siblings and the children of your siblings.
Anyone that shares your blood line. Parents, siblings, cousins, aunts, uncles. Even if its a second or third cousin the blood line is still there. If you are having a relationship with someone in your blood line children would be very risky. The closer the blood line (sister, brother, cousins) the higher the risk of birth defects.