What I want to accomplish:

Every time the stage is (C or D) and a field named "Date of Payment" is being changed from empty to ANY VALUE, I want to first validate if a third field called Buyers Name is filled in. If Buyers Name is filled in, then the user should be able to fill the Date of Payment. If the Buyers Name field is empty, the user should not be able to fill in "Date of Payment." How can I accomplish this??

It looks like when I try to create a blueprint I can only select a field that is a picklist field. Why can't I select my date field and run a workflow from when it is empty and then someone wants to fill in any value there?

Please help