I suspect that most anyone who makes it that far in the "study" is already sufficiently indoctrinated that the relatively remote chance that they'll need a blood transfusion allows them to dismiss any questions they might have regarding the implementation of the blood ban. They've already invested so much time into the cult that their CD will prevent them from raising questions at that stage. Combined with the social pressure of having 2 JWs explaining this to you like it's the moist rational thing ever it's probably only going to raise questions of they or a family member has needed blood in the past.
The blood ban in general I'd probably losing them recruits, but nowhere near what they're losing just by being exposed for what they are on the internet.