Can't script filling a transparent layer mask with full black

Pushing levels all the way right, and I still am left with pixels. And, I need something actionable/scriptable regardless of the type of transparency I come across. Any other thoughts?  

The selection was derived from Layer Copy 1, then expanded and smoothed by 20px, then inversed. The active layer is the white fll 255,255,255 layer, made with the following code: 

function newColorLayer(red,green,blue, doc) {
    newLayer = doc.artLayers.add();
    newLayer.name = red + "," +green + "," + blue;
    newColor = new SolidColor();
    newColor.rgb.red = red;
    newColor.rgb.green = green;
    newColor.rgb.blue = blue;
    doc.selection.selectAll();
    doc.selection.fill(newColor, ColorBlendMode.NORMAL, 100, false);
    doc.selection.deselect();
    newLayer.move(doc, ElementPlacement.PLACEATEND);
};

Anything else you want to see?