Is he a medical professional? Yes
Will the needle go into his own vein? NO, see below
Will he put someone else's blood into his own vein? No
Does he have a tube of different blood in his sleeve? No, but...
Beneath his skin? ...YES.
Does he have chimerism? No, but you've got the
Dr. John Schneeberger ("Whitley" in the puzzle) had passed two DNA tests after a patient accused him of sedating and raping her, but the police had him submit to a third test years later, after the patient maintained the accusation. At that test, the forensic technician (our "Dorian") wanted to take blood from his fingertip, but Schneeberger refused, saying he had a disease that would cause his fingers to bruise. Instead, the blood was taken from his left arm, where the two prior tests had drawn blood; the blood was deemed too degraded for testing.
But after Schneeberger's stepdaughter also accused him of sexual assault, the police (with a warrant) took DNA from all over his body, which matched. Schneeberger later confessed that he had surgically inserted a tube of blood from one of his patients under the skin of his left arm, which is why the three earlier tests did not match his DNA.
I saw this on an episode of Forensic Files, which I've been watching almost non-stop since GalFisk's puzzle about the ricocheting bullet. https://www.youtube.com/watch?v=ABK6L0oHbbw
Nice job everyone!