Why is the term "bloodline" often used instead of "family tree"?
edit: The reason I find it an odd term is because human ancestry literally doesn't follow a line. It always branches off, even if only to just include two parents. It's a tree like structure, a line would misrepresent it
So if you look at a family tree, the bloodline is the direct order from person a to person b, with everyone in the middle. It doesn't include everyone else that isn't in that direct path.
It doesn't include brothers, sisters, aunts, uncles, marriage or anything like that.
Bloodline is a subset of the data in a family tree.
Whichever path it takes. It will only go in a single path unless you have some incestuous relationships. And if that happens and multiple routes work, it doesn't matter which one you take.
It would generally be between a person and a specific ancestors of theirs, so that depends on who is is tracking towards. Often it will be qualified with something like "Paternal Bloodline" or such, in which case it would follow the father, the father's father, the father's father's father, etc. Or for royalty, it would track from some historical sovereign figure and follow their legitimate heirs down to the individual being examined.
I suppose it is in a fashion, but not necessarily. Let's say you know you have a ancestor that was part of the first expedition to the arctic. The line of ancestor to descendent between that person and you would be the bloodline. Everyone you are related to would be your family tree, but that could be hundreds of people depending on how far back you go, and could be thousands of people if you start looking at everyone descended from that person. But you are only concerned with the direct line of lineage between them and you, and that would be your bloodline.
It's just different use cases. A tree would show relations to the individual, a line just proves they descended from a particular person. Applications of it might be a bit outdated, but I don't think there is any more reason to show relations in a tree than "oh, that's neat".