Do you have access to the SQL? Can you check the tables to be sure the values are there?
You can use [DocumentCreatedWhen] which would be the page create time. But if you want to create a field to select some other date instead of the page creation date, it should be available as well.
Oh wait, did you specify the columns in the repeater that's pulling the data? Did you add "BlogPostDate" to the columns list?