While you could argue (and I'm sure the writers would) that HRG doesn't know Claire's blood brought him back from the dead (He was dead - a bullet through his eye pretty clearly killed him; and Claire never asked how he wasn't dead, and HRG apparently never learned regenerators could heal others in years of hunting), Peter definitely knows it can heal people because he saw Adam's blood heal Nathan's burns. Angela should know, too, since she knew Nathan was burned and then wasn't, and probably should have dreamt it.