This was essentially my take on it since the real answer in mind is that the writers' never settled on canon for Sam's freaky powers and I tend to find that annoying. He used his powers sans demon blood early on when under duress so I've decided to handwave it in essentially this way so that I can still be happy. De-nial FTW! Also, maybe the demon!blood focused him? And addicted him so that Sam was Ruby's bitch for S4. Poor Sam. *pets him*