Field of celestial blue color symbolizes the common sky of South America. The Southern Cross represents the course and the commitment for common destiny. The red stars each refer to one of the states of South America. The red color symbolizes the national passion of each one of these states, that must feed with their fire the Union - all aligning itself around this common objective.
The pale blue flag shown above is presented as a floating graphic and labelled "our flag" on a page entitled "Foro para la Proyección Polítical" (Forum for the Political Project of the South American Union). Therefore, I assume that the defunct website was more a lobbying project than an official one and that the flag was a project, too, which might never have existed in real.
I believe that this flag for the South American Union (UNASUR) was merely one personal design out of many that have been individually designed.