When I upload my SVG to Cricut Design Space, it's all black. What happened?

Most likely this is because your objects stroke color is set to black. So if you have a black stroke and no fill color, Cricut Design Space will use the stroke color you set for the fill color too. And since it uses black for the outline of all objects, it appears as though you see nothing but a black box. Everything is there, but you can't see it because it's all black on black.