Contents, Flags, Users and Node views
As a workaround you could show a list of accepted challenges for each user and link them to node/add/blog with the title prefilled using the prepopulate module.

You could also create a node reference from blog to challenge, use a "select" widget and narrow the option with a view (that's a cool feature of the node reference module). In that view you limit the records to the current user and only the challenges they have flagged.