They’re different things.

A family tree is a representation of your ancestry by tracing backwards (usually, some people use the term for anything related to family ties). It’s backwards in time, almost always.

Your bloodline is forwards in time from ancestor. The idea is that there is a clear line of descent from one person, or a small group (depending on how it’s being applied in context).

Think of it in terms of race horses.

Secretariat had a family tree of horses before he came along. He had a dam and sire. They had dams and sires, and so forth. The tree, when laid out, may include siblings of secretariat, but wouldn’t include “nieces and nephews” under normal circumstances because that’s not really the point of the family tree as a term/idea. That steps into general genealogy.

However, from secretariat, you can trace records of horses descended from him, and that’s literally his bloodline. That’s his genetic line where his semen was used to make other horses.

Unlike horses, you couldn’t guarantee paternity for humans until genetic testing came along. At best, you could exclude someone via blood typing, or some inherited features (like a cleft chin).

The term bloodline itself started before knowledge of genetics was a thing to any serious degree. Mendel didn’t do his thing until the 1800s, and bloodline is a compound word that goes back 200 more years. But it is related as an idea. Related being the key word to that.

To reframe it, I have a family tree that includes a wide range of ancestors going back to Europe before we can’t find anything on either my matrilineal, or patrilineal side. Both my father’s surname and my mother’s maiden name have been traced back at least as far as the 1700s. However, my “bloodline” descends from the oldest known ancestor, a man that had a different name because it was in German instead of being anglicized. It also descends from multiple other people, but you could trace each of those and determine who else shares that bloodline.

Me and my sister are the only living people that have the exact same family tree, but we share any given bloodline with thousands (at least) of known individuals.

Could be to heighten the importance, but they are not exactly the same thing, as one is directly genetic. You may see the term used when talking about kings and queens.

Imagine a line running only down the tree connecting two individuals - that’s a bloodline

If you can draw a bloodline from one person to the other, they are of the first’s bloodline. Your full blood siblings are not in your bloodline, though you share all of each other’s bloodlines

Because theirs isn’t actually a tree.

It sounds cooler. I don’t think it gets more complicated than that.

When I hear “bloodline”

A tree is a terrible representation since people will appear in more than one place on it. brother sisters marriages (which did happen) tend to produce deformed kids, but first cousins have good odds for normal kids. By third cousin odds of genetic issues was close enough to zero, but those kids will have six great great grand parents not the mathematical eight. I didn’t mention half siblings but that happens too and a couple generations below could marry safely.

the above isn’t just theoretical. Before modern transport you often lived and married in the same village for many generations. It would often be impossible to find anyone to have kids with that wasn’t at least sixth cousins from more than one path.

Who is actually using this term? I’ve only heard it in like medieval period fiction.

If I heard anyone start rambling about their bloodline I would immediately start to wonder if they were a fascist.

Due to increased narcissism within the population