When I upload the file to my design software, all I see is a big, black box!

The issue here is that Cricut Design Space will fill in any empty boxes (boxes with no fill color) with whatever your outline color is. All you need to do once you upload it to Cricut Design Space is select everything, click Attach, and cut it, and it will cut out fine. If this bothers you, change your outline color to something other than black, save it, and re-upload it.