Currently only regular wiki pages can be used as source of information when an excerpt of the page content is wanted to be shown on another page or blog post.

When excerpt macro is placed on blog post and excerpt-include macro on page/blog post, it ends finally to error message "The page <name> does not exist."
When setting the blog post as the source of information via excerpt include macro dialog / wizard, it finds the blog post and it can be selected. But still the same error message occurs when I checking via Preview tab or after saving the page.

It would be highly appreciated, if also blog posts could be used as source of information. Since the excerpt macros are supporting regular wiki pages as source, the assumption is that implementing the support for blog posts should be quite small.

Support request of this case is also sent to answers.atlassian.com:
https://answers.atlassian.com/questions/98956/how-to-fetch-part-of-blog-post-to-page-blog-post-in-confluence

Workaround

Use the Blog Posts macro instead, and use labels and the parameters on the macro properties to show the post you want