The marks/scarring thing is real. I need regular blood draws and infusions (iron deficiency anemia), and I look like a drug user just because my elbows always have marks, and I'm pretty sure the left one is scarred after I needed a blood transfusion a couple months back.
As somebody who has now needed a blood transfusion, thank you, donors! Please donate if you can. In addition to people needing them for injuries and childbirth, some of our bodies are just shit at making our own blood.