How to display one block among two block in Drupal? I have two block (like Block1 and Block2) and I have to display one block among two. Condition is Block1 will be display into the whole site if Block2 is not present there. Block2 will be displayed into specific pages which is controlled dynamically, i.e. when user create content in future they can control the Block2 display. So I need to display Block1 where Block2 is not present.

I am using Durpal 7.

Note: I have tried with default block visibility but this is not worked for me either.
This sounds like you're going to need to use something like Context to toggle the display of each of the two blocks depending on the context of the page.

See this tutorial for more info on how that works.