Freezer paper is used extensively by quilters for a variety of purposes. I use it mostly for machine applique. Some people use it for paper piecing. It is also used for making compass stars (complex patterns that require accuracy).
Iron the shiny side of the freezer paper to the wrong side of your blocks. It peels off easily and should not interfere with your interfacing. I am assuming the interfacing was ironed to the t-shirt material, so the side that you would iron the freezer paper to should not have fusible on it.