How to blur edge of a video clip placed on top of a photo background

Hello, 

I am new to Adobe Premiere Pro. I have a question about the blur an object edge. 

In my video, I used a photo as a background and added a video clip on top of this photo. The video clip was placed in the middle of the background photo. I would like to blur the edge between the video clip and its background photo. 

I tired the Fast Blur technique which I saw the tutorial in the Youtube. But what I got was that the background photo also got blur. This is not what I want. How can I only get the edge of the middle video to be blur but background photo is still clear?

Is there any tutorial video for this?

Thanks.