when blob cache must be invalidated by framework ?

As a general rule any kind of change in file content or permissions should force the cache to be rebuilt.

When a change is observed , file is removed from indexes.

Operation	Effect on BLOB Cache
Editing a file that is BLOB cached	The cached file is removed from the cache.
Deleting a file that is BLOB cached	The cached file is removed from the cache.
Deleting a file that is not BLOB cached	All cached files in the list containing the file that was deleted will be removed from the cache
Deleting a folder in a list	All cached files in the list containing the folder that was deleted will be removed from the cache
Renaming or Deleting a list	All cached files that were in the list will be removed from the cache
Making any changes to a list ·         Permissions ·         Title, or Description ·         Adding or Removing Content Types	All cached files that were in the list will be removed from the cache
Renaming or Deleting a web	All cached entries in the web are removed from the cache
Adding or removing permissions to a web, changing inheritance, adding, updating, or deleting roles	All cached entries in the web are removed from the cache
Deleting a site collection	All cached entries contained in the site collection are removed from the cache
Modifying User Policy on the web application	Entire cache is abandoned, a new cache folder is started

Ref : http://technet.microsoft.com/en-us/library/ee424404(v=office.14).aspx

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