To the best of my knowledge, the only way would be to have control of the page being loaded into the iframe, and then setting that page's background to the desired color. Otherwise you might have to look into some server-side scripting option to grab the page contents and parse/massage it to get the desired look.
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation
As long as the web page the iframe is loading doesnt have a background set, and the iframe doesnt have a background set, it should be transparent by default.
I added an iframe and loaded an html page with 5 paragraphs, and nothing else. If I were to set a background color on the body of the page the iframe was loading, then the background color would display instead of the transparency.