Installing a simple drainage and impermeabilization system behind a block retaining wall
The only application I am familiar with where you would deliberately create an impermeable barrier on the uphill side of a retaining wall would be one where you had hazardous material on the uphill side, such as sewage or chemical waste. Normally you allow water to percolate through the block retaining wall, although if the wall is high enough, you may also install drainage tile on the uphill side of the wall near the base.
Since you provided no details of the design, I have no idea what you are trying to accomplish, and can't help you decide if your idea makes any sense. How about posting a sketch of the design, and some details like height of wall, type of block etc.