what are blood lines?

Answer 1

Bloodlines are chains of parentage over time. A dog's blodline would include his father, mother, their parents, their parents parents, etc. It can also be used to refer only to the father (and father's father etc.) of the person or animal in question.

Answer 2

Blood lines are the descendant history of Horses, or people. It tells you how pure the breed is{horse} who was the stud and the Meir were. The family tree of the animal in question. Or as rich people being Blue Bloods.
Blood line is the family tree! Just another way of saying the same thing.