I'm rocking the denim jacket under a blazer today! Bit of a struggle getting it on, but once it's on, it looks quite good and my blazer isn't pulling at the buttons.
I've rocked my denim jacket under my peacoat, dunno about it with a blazer
Men’s Style 101: The Denim Jacket, Re-imagining a Menswear Staple « The Boy With The Thorn In His Side
I wrote a blog entry about it if anyone's keen to see the result. Feel free to read my other fashion stuff!
Double Denim Jeans works if the colour's match! lol
But seriously, that is fucking horrible.
I think that's a good litmus test though - if your Double Denim could be considered even remotely similar to that, you're doing it wrong.