InDesign can show color separations. If you select this, you will see that your image 'continues' below the black bar. That is, the bar over the image is not knocked out when you deselect the Black plate.
For better results, you need to make sure there is 'nothing' below the black bar (I think you need to create a separate black swatch for that, as InDesign by default overprints its own black), or change the overprint-black setting in Preferences.
For the very best result, create a new "Rich Black" color swatch and use that instead of the plain black you have here. (http://en.wikipedia.org/wiki/Rich_black)
The printer didn't really meant to say your black is transparent; rather, he literally meant the ink itself is transparent. Your Solid Black is not as dark as, say, Solid Black with Some Red and Blue in it (that's the general idea of Rich Black). Because the original photograph is printed in color "below" the black bar, it's possible to see nuances of black in that bar.
As Jongware says
And all inks are transparent, even black ink. So a rich black is necessary
See this blog post for indepth stuff
And the comments afterwards
Thank you, i have changed the colour of clack to one with red and blue in it as you suggested.
He is also saying " i think the black box needs to be set to knockout rather than overprint" - i am not sure how to do this though??
You don't need to do anything—the default preference for 100% [Black] is to overprint, all other colors and swatches knockout, including your rich black.