Special:BlankPage is used by a number of user scripts to create custom interfaces. When a user that doesn't have the relevant script installed, the link makes no sense. Adding a "help" link targeting https://www.mediawiki.org/wiki/Manual:Special_pages#Custom_special_pages would be helpful for such users.
Adding this help link will also allow script developers to change the target to their own help pages, rather than needing to add their own help link functionality. A snippet as simple as $('#mw-indicator-mw-helplink > a').href = '/wiki/Foo'; would be enough to retarget the link.
I'll leave this open for a couple days to see if there are any objections to adding the link.