This is a Matthews 8x8' Solid Black Overhead Fabric. Also known as "Rags" these are the materials that are tied to the Butterfly/Overhead frame.
For many years, only basic materials were used, such as scrims to reduce light, silk to diffuse light and solid to eliminate light.
More recently many new materials have been added to allow many degrees of diffusion (Grid Cloths, Black Silks, etc.) plus many new reflective materials are now available to make large reflectors for both sunlight and artificial light such as Matthflectors, Grifflectors and Lames.