I am creating a document in InDesign CS4 that has black as a background and another image, placed from Photoshop that also has a black background. When I print the document the two blacks come out different shades and you can see the separation of the two items. The black background is CMYK K100%. The placed graphic is C 75% M 68% Y 67% K 90%. I really don't know what to do in order to match up the colours?
You can specify the black background in IDCS4 so that it is C 75% M 68% Y 67% K 90%. This is a "Rich Black", since the K plate is suplemented by CMY. Defining a solid black as 100K usually looks a little weak, so when a solid black background is desired, a Rich Black is usually specified. The above combination adds up to 300% ink coverage, which should be okay for a Web or Sheetfed press, but I wouldn't exceed 300.
Also, make sure that both Photoshop and ID are set up using the same CMYK color space so things don't get changed accidentally. I'd also make sure the Photoshop image is set to Relative Colorimetric rendering intent with Black Point Compensation turned on. If you use Perceptual, you may get a tonality shift.