Waldemar,
I suggest you to review the Agile ASAP methodology. It provides clear description of how to do visual blueprinting. It definitely does not leave the scope definition or blueprint open until Realization stage. Even in pure SCRUM projects you start with definition of the backlog which is exactly what we do in the front end of the project.
We do either Scope Validation (in case we have a good baseline build that we can start with) or Lean Blueprinting which has objective of completing the backlog of work for the Realization stage. Where we differ is that the full design work is not done in Blueprint, but rather some of the more fine elements are left for Realization. This gives the team ability to better react to changing requirements resulting from end-users (product owner) understanding the solution capabilities better. We also recommend to use value based decision making about modifications or enhancements.
I assume that your main concern is scope management after the backlog is completed. This is especially important in relationships where customers contract companies to deliver the solution. In my mind the fact that the methodology is open to product owner introducing changes does not preclude the contractor from adhering to the contracted scope and handle any out of scope items as scope change requests.
