The IDBOpenDBRequest.onblocked event handler is the event handler for the blocked event. This event is triggered when the upgradeneeded should be triggered because of a version change but the database is still in use (that is, not closed) somewhere, even after the versionchange event was sent.
In the following example you can see the onupgradeneeded handler being used to update the database structure if a database with a higher version number is loaded. For a full working example, see our To-do Notifications app (view example live.)