It isn't clear what you're trying to do, or what's preventing you from doing it.
I'll start with the assumption that 'BLURB book' is a commercial printing service that will produce a physical book from media you provide.
Second, you don't explain what's preventing you from just printing the Keynote presentation as PDF and sending that. You allude to some issue with page size, but don't clarify what that is. A4 is not 16:9, BTW, not even close, all A* paper sizes use the ratio of 1:1.414).
Is the issue that your presentation is based on 16:9 and when you print it on A4 you get a border?
if that's the case, you can edit (a copy?) of your presentation and change the 'slide' size to match a sheet of A4 (210 x 297mm) via Inspector -> Document. This will expand/shrink the canvas, and you will likely have to manually adjust (or at least check) each slide to see how its content fits the new dimension.