BLANK Corner Color

[sjtrny]

Describe the bug

There are two related issues related to BLANK corner colors:

1. calling paint_corner with color BLANK fails because it is assigned an empty string and inside paint_corner the color string is checked against None.
2. the blank corner color string is hardcoded and scattered throughout the library, this should probably reference an enum value which is set in one location

Expected behavior

Calling paint_corner(BLANK) should not throw an exception since BLANK is a valid color code.

Additional context

Created issue to track TODO items arising from #71