He 'blood packed' taking a transfusion of blood that had been previously extracted from his body during his best training phase.
Bill Rodgers himself said Viren 'cheated' in so many words. I heard him say that and was somewhat surprised he did. Bill did not get specific, merely using the word 'cheated'.
Blood 'packing', or loading the body up with additional hemoglobin or red blood cells, can be argued as NOT being an illegal activity, as you are merely placing a natural substance back where it came from...that is if you have a shifty enough lawyer.