Here is another way to try to darken the corner: select the area out loosely, then copy and paste to a new layer, then change the layer mode to Multiply. (Near the top of the Layer palette, click on Normal and select Multiply) When you do that the layer will darken depending on the opacity chosen. Using a Paint brush, foreground set to black, set to Linear, and say an opacity of 50%, paint over the selection to darken it more. (Or lighten it with the foreground set to white.) Set at 100% opacity would get you to black.