I think you mean direct descendant, which means the children, grand-children, great-grandchildren, and so on of an ancestor. No aunts and uncles, no in-laws. Collateral or linear descendants include aunts and uncles, cousins, etc.

"Blood line" (only used in some parts of the world and not an accepted genealogy term in many places) means anyone with a direct (blood) connection, such as parents and siblings.