I've done this on a video, but for those who missed it.
When burning in a background to make it black create a curves adjustment layer first and adjust the curve so the image looks incredibly light. From this you will see the areas that are already black. Click on the layer below that you want to edit and use the burn tool to make all the areas you want black become black.
Advanced users could create a second layer under the adjustment layer and edit that so the original is untouched. You may also prefer to do the burning in using a layer mask. If it alls sounds unclear check out the video I created here: Using Photoshop Adjustment Layers as a burning preview