Let me say right up front that Custom segments show session data sets and not single pageload data in Google Analytics.
How not to do it:
Hypothetically speaking you may want to setup a quick reference to display just your blog post traffic (excluding the blog homepage and archive pages). So you might think that you could setup the following custom segment to show pages containing "/blog/":
But then draw the conclusion that "custom segments are broken in Google Analytics" when you are presented with the following results (note the inclusion of the /index.php and other non-/blog/ related pages).
The reason for this unexpected behavior is that this custom segment will display all page loads from sessions that include any blog visits (in this case) - Providing you with a general traffic pattern for blog visitors which may not be exactly what you are looking for.
How to do it:
If you want clean page load data that only contains your blog post pages then you should make use of the "advanced filter". Often easily over looked - it is just to the right of the simple filter at the bottom of the page.
The advanced filter allows conditional filtering which enables you to setup a highly customised data set.
Now you can include blog related content and exclude additional pages as required to create a precise custom data set. This makes it simple to extract the exact data that you are looking for.