I'm just posting to follow this topic, because I don't think there is a way, but sometimes I am surprised by an answer.
But as far as I know, you don't have that kind of granular control with your blocks. If you add a block to your Pages, it shows up on ALL those pages. If you add a block to Gallery or Downloads, it shows on ALL subsequent links under those apps. Likewise, any block on a topic shows on all topics, in one forum shows on all forums, etc.
But, I am eager to hear I am wrong, so I will wait in anticipation 🙂