At the time of execution or planning of user story there can be one or more impediments blocking the further progress of story. If there are any such impediments blocking story progress, you can block story to notify the scrum master. Once you have marked story as blocked, scrum master will look at the impediment(s) and try to resolve to the earliest.
To Block a story,
1. Click on Block icon next to the Status field.
2. Note down the reason (Impediments).
3. Click on Block button.
As soon as you will block a story, it will be marked red.
Once all impediments has been resolved, you can unblock the story and mention the reason for unblocking the story.
To Unblock a story,
1. Click on Block icon (It will appear in red colour)
2. Mention Click the reason for unblocking it.
3. Click on Unblock button.
As soon as you will unblock the story, it will again appear in normal mode.