This card isn't going to answer that question. Sorry!!!
Why? Because I'm not sure of the answer. I've never personally used talc because I've heard too many times that it isn't good for you feet, causes more problems than it's worth, etc.
But this post I linked seems to be different. The guy had well fitting boots and doesn't really get blisters, but, rather, faced a problem with his feet getting sore because they were soft and damp due to his feet sweating.
So, he tried to use talc. He stopped every once and a while, took off his boots and let his feet cool down, then covered them up in talc powder again before putting on his socks and continuing. And he saw a lot of success!
Here's his advice:
1. Make sure you have a decent pair of boots
2. Take plenty of decent hiking socks
3. Change your socks when they get damp (if you do this as early as possible you have a fighting chance to dry them in your sleeping bag)
4. Regularly let your feet rest and breath
5. Regularly apply talc to your feet
6. If it is raining or very damp, wear gaiters to stop water getting into your boots
What do you guys think--talc or not talc?