Digital Light & Color

Is there a way to use an oval gradient transform to darken the edges of an image instead of lighten? Like an option to invert the mask within that transform?

You control the gradient by editing the color line at the bottom of the gradient dialog box. The color line lets you specify a set of colors spaced at adjustable locations along the line -- the line is then mapped to the radius of the gradient so you can produce a variety of effects. See the help file or manual for instructions on using color lines.

Also, if you enlarge the input image window, you can pull the oval outside the limits of the image if necessary.