I have the following blue filled path that leaves a small portion of the artboard empty. I want to know if there's a way to select the inverse part of the artboard (the white part) and turn that into a path as well?
No. Not directly.
But...
- Draw a rectangle filled with any color other than blue, to cover that bottom portion
- Move this new rectangle behind the blue
- Select both shapes
- Hit the Merge button on the Pathfinder Panel
You'll end up with 2 shapes.