I've never used a Blackburn valve..Are those the same as HydraPak? I know BlackBurn owns Hydrapak. If it is a Hydrapak valve, they are supposed to be a real pain.
It seems like you could switch bladders, just make sure they are of the same capacity. Obviously if your pack is made for a 70 oz. bladder you don't want to switch with a 100 oz bladder.