Eh? Did you actually see the bullets that were used? and regardless of which types of bullets were used (as was pointed out earlier in the show itself at the very close range a blank could still kill or at least do some nasty damage) the whole point was that he could predict which chamber the bullet was in........which he did.
As an ex-army cadet instructor i can tell you a blank is potentially lethal up to about 50 yards - certainly at point blank it will blow a nice mess into the side of your head. (Though there's no bullet warhead, there is cardboard, paint and other chemicals in the front of the blank [packing], which is expelled in the explosion)
Sft42 has it right, and I think the point is worth repeating: the crux of the trick was Brown's ability to determine which chamber the bullet was in. The 'illusion' was that he could read the volunteer's mind. Whether the bullet was live or a blank makes no difference.
I assume Xyzzy that you didn't actually see the programme as Derren's stunt (note that I don't use the word 'trick') was nothing like the one you have linked to. If Derren had to use a blank for legal reasons, so what? It would still have been fatal if he'd got it wrong and it was still a good exhibition of his abilities. The guy's fantastic.