Within the past year Pivotal Tracker added a feature that allows you to mark stories with blockers. These are visual indicators with a description that are used to show a particular story is blocked, that is, it cannot be completed until something else is taken care of.
In order to maintain the health of the project, it is good to triage these blocked stories from time to time. The best way to identify all of the blocked stories is to filter them into their own column.
Enter is:blocked into the search bar to show all of the blocked stories.