I was curious how few black cells were required for a Kakuro of this size, and also how many sequences of length nine I could fit in it. Provided that all sequences have to be of length at least two and each white cell is part of two sequences with a clue (even if replaced with a '?'), you can prove this puzzle is optimal in both respects. If you would allow sequences of length one, the puzzle can in fact be modified slightly to have just four black cells on the main diagonal, allowing ten sequences of length nine, while it could be based on the same solution.