That first PostProcessBlendWeight is kinda odd, not sure why do you need that. All that will result is a white pixel when you set it to 1.0. No reason for that.
The second image have no context. The A input should be the SceneTexture:PP0 input from the first image, the B is the color you want to apply at the white edges, then theres the alpha channel which is the sobel edge output and you should clamp it between 0. and 1. to make sure no value larger than 1.f will be coming into the Lerp’s alpha. Then it should work, unless the sobel extracting is also messed up in which case you’re better off creating a very simple 3x3 kernel on your own, then apply the two sobel edges (summary of right and bottom sobel), that should work.
Here, the custom node is a little HLSL code i wrote that will sample 8 pixels from the neighbours and summarize the sobel (also applying a min[1.f,x]) the way i just described. The floor node will make a very narrow high pass filter (only allow values of 1.), but you can use a pow() node as well that will give broader spectrum for more edges.