To build a blueprint effectively, it is important to gather sufficient information to gain an understanding of the requirements and the scope of the required solution, but not so much information that you end up in detailed requirements gathering. A good rule at this stage is to limit the requirements to single lines that outline the intent and the benefit of the requirement. A business intelligence and data warehousing solution can be complex, so having a map that outlines the journey is vital. The journey for a business intelligence and data warehousing solution is as important as the destination.
The solution should take into account that requirements will evolve through the project lifecycle and the solution will need to be flexible enough to accommodate these changes. It is very likely that through the iterative process of designing, developing the solution requirements will not only change, but be invalidated during the lifecycle.