Sounds like your [Paper] swatch is set to black. You can colour your paper with this swatch and preview your design on coloured paper. If you set the [Paper] swatch to a colour (like black) it will only show up on your screen and never print.
To fix this. Double click on the [Paper] swatch and set all ink percentages to 0%. Then add a new layer with a new rectangle covering the whole page. Apply the [Black] swatch to this rectangle and move the new layer to the bottom.
