Su Doku
Su Doku
[A square divided into 2×2 squares, the top-right one has an 1 in it, the bottom-right one has a 0, the two left ones are empty.] Binary Su Doku

Su Doku (Japanese for "single number," and now usually written as "sudoku") is a type of number puzzle, in which the player must place digits in a matrix field in the correct arrangement, such that they do not repeat within given domains. The most common arrangement is a 9×9 grid subdivided into nine 3×3 grids, into which the nine non-zero digits of the normal decimal counting system must be inserted, with no digit being allowed to appear twice in a horizontal or vertical row or in each individual 3×3 grid. The number and combination of pre-filled squares determines the difficulty of the puzzle.
However, Randall presents a 2x2 binary sudoku puzzle which isn't subdivided. The joke is that the binary system has only two digits (0 and 1), and as a result binary sudoku puzzles would be trivially easy and thus pointless. The puzzle in the comic would be completed by filling 0 in the top-left and 1 in the bottom-left empty box. The only other possible grid would have the 0s and 1s swapped. This fulfills the criterion of having no repeated digits in any row, column or cell.
The title text appears to reference a series of published sudoku puzzle books called "Martial Arts Sudoku". The difficulty of each book is denoted by a martial arts belt color, with each color representing a certain skill level. A red belt is a rather high level, second only to the black belt. When applied to binary sudokus, a sudoku with one number given would be the most difficult one (though still trivial) and thus be a black belt. This sudoku has two numbers given, hence the medium red belt level.
[A square divided into 2×2 squares, the top-right one has an 1 in it, the bottom-right one has a 0, the two left ones are empty.] Binary Su Doku
