The black stuff on flex circuits is actually dual purpose. Stiffener and also EMI shield.Flex circuits in Consumer Electronics is typically used in Two places. Camera module interface and Display interface both of which push serial data to upwards of 1GB/s LVDS(MIPI) CSI and DSI. And due to tlack of dedicated ground planes( cheaper) they add a adhesive that serves to contain emissions from the 8b/10b(Manchester) encoded data stream.