Small update,i made a mistake with the last noodle screenshot.The black value is not from the spec map,instead its from the Diffuse color map.
Based on Allegorithmic PBR Guide
as you can see,the black parts on the diffuse are these parts that gets metallic.
Since in the specular workflow (all based of the PBR Guide),the Metallic color are taken from the spec map,i simply added the color from the spec map to the diffuse map,if the diffuse color is pure black(0,0,0)
