Blurred edges of roipoly

I am using the below script to overlay two images. I create an ROI so that the output shows one of the images but with a cut-out revealing the other image. Can anyone suggest how to make the edges of the ROI cut-out softer/blurred so that the ROI blends better with the other image?

clear all
rgb = imread('Image1.png');
imshow(rgb);
BW = roipoly(rgb);

rgb = imread('Image1.png');
imshow(rgb);
I = imread('Image2.png');
hold on
h = imshow(I); 
hold off
set(h, 'AlphaData', BW);

Answer 1

[Image Analyst]

You could blur the overlay image and then replace the overlay image in a neighborhood of the edge of the BW with the blurred version. Try it.

[Jessica Yorzinski]

I understand how to blur the image, using something like this:

windowSize = 30;

kernel = ones(windowSize) / windowSize^2;

blurredImage = imfilter(I2, kernel);

But, how would I place this atop the edge of BW?