Your descendents are your children, their children etc.
Your parents are your ancestors as are their parents and their parents.
I do not use the adjective "direct". The only ancestors you have are you parents, their parents and their parents etc. Anyone else is not your ancestor but a sibling or cousin of your ancestor. You do not have any "in-direct" ancestors.
When someone says they are a "direct descendent" of someone my thought is - you are a descendent or not a descendent. It is not necessary to add the word "direct".
Re "blood line" - not sure what that means.