Formal or legal term for a person related by blood, as opposed to in-laws or adopted family.
He is not my blood relative.
The inheritance is passed on to blood relatives.
肉親 refers to close blood relatives like parents, children, and siblings, with a more emotional nuance, whereas 血縁者 is a neutral, legal term.
血縁 (blood relation) + 者 (person). A straightforward kanji compound.