An airfoil used in a turbine of a gas turbine engine, the airfoil having a blocker insert formed within the serpentine cooling passage of the airfoil. The blocker insert forms a cooling air passage between the serpentine passage within the blade and the outer surface of the blocker insert. The blocker insert is formed of a carbon/carbon composite material and is cast into the airfoil when the airfoil is formed. A ceramic layer is applied over the blocker insert to produce a composite insert prior to casting the airfoil. The ceramic layer on the insert is then leached off to form the finished airfoil with the reduced size cooling air passage within the airfoil. The blocker insert can be formed with a cooling air passage therein in order to provide additional cooling for the finished airfoil.