I had a few issues with getting external content to open in an iFrame. The problem was always that the iFrame size was fixed across the different devices. In the end, I used a little trick to get things working correctly. I don’t know if it will work with your Blog, but this is what I did.
I inserted a video bric where I wanted my iFrame content to appear. I then edited the code in the sidebar and changed the source bit to point to my external content. This gave me the expected result - fully resized in each device view. You can see it working at http://triselcommunications.com. Just scroll down the page to the Digital Publications Section. You will see a page-turning publication displayed in an iframe. Move further down the page to the Online Presentations section, and you will see a keynote presentation displayed in an iFrame. Both of these were created by using a modified video bric.
Of course, for this to work correctly, the content to be displayed in the iFrame must be responsive - otherwise you’ll still be faced with having to scroll from side to side.