I may have the dates wrong on this and I can't remember where I saw the interview, but didn't Contador have a cold/flu between the Dauphine and Tour, for which he took antibiotics. If he withdrew blood for future use during this period then surely any blood tested during the tour would show traces if he had transfused.
Is it possible, in only a three week period, to have a course of antibiotics, a course of clenbuterol, enough to be effective, then withdraw blood and be sure it is taint free. The risk must be appallingly high as I understand GC contenders are subject to more frequent testing in the run up to big tours.
Also would you withdraw blood if you were unwell. Don't know.