First of all, if you just did it, the odds would be very slim that anyone would even know. I doubt there are any JW's that work at a blood bank.
Second, if someone did see you, you could just say that you were giving it for one of the acceptable fractions such as for hemophilia or some other "acceptable" use.
The worse that would happen would be that they would tell you not to do it anymore since you may stumble someone.
Rub a Dub