Photoshop :: Custom Frame/border In PS

I've been trying to make a custom border for my images in PS. I already have an action that does this for me:

Back when I printed images myself in the darkroom, I loved FF images and the black edge they have. Here's an example of one of my old prints (scanned):

how to get a somewhat soft black edge like that that isn't perfectly straight? I've found a special brush in PS:CS3 that gives me this:

Now my issue is I need this to be done in an action. That was done "free hand" by drawing a straight line across each edge, then adding the white after with the canvas size tool... Also that brush isn't perfect. I'm looking for something that has that perfect looking blend/rollover as it transitions into the image.