Bolide Movie Creator does not have a "background". Black color in areas of the frame most likely means emptiness - the absence of visual blocks in this place of the frame/timeline. If you do not want black color there - add a visual element - photo or video to the track UNDER the cursor. Tracks in the program - like layers in Photoshop, are stacked from bottom to top. Having placed the necessary background in the lower track, it will "appear" on the empty areas of the frame. Another method is suitable for when your visual element does not take up the entire frame area (for example, a vertical video in a horizontal frame). In this case, you can right-click over the selected visual block and select the "Blurred background" pop-up menu item.
We should also mention the background for text blocks. Pay attention to the "Background color" tab in the text block settings dialog. Everything you are looking for is there :)