I agree with the subtle appoach.
Also advice to everyone: Blood policy is probably not the strongest argument these days. There are many non JWs and ex-JWs would refuse blood. The US army has been pioneering non blood surgery for the battlefield and there are numerous hospitals now that specialise in non-blood surgery. Here in the UK we get news stories every 3 or 4 years showing the results of studies that have found JWs to recovery quicker after heart surgery, cancer surgery etc. Yes the JW will be ready for your argument and will have plenty of secular evidence to back up and "prove" the witnesses were right all along.
If you must talkk about blood its probably better to concentrate in the flip flops over blood fractions etc rather than the "JWs would rather die than have a life saving blood transfusion" argument.