Case 1: The existing block is the first clue
If the block is the first clue then everything before it is white (our background color).
Case 2: The existing block is the last clue (black3)
If the block is part of the last clue then we know that it MUST be preceded by at least one white square and one black square before any red squares are filled in.
In any case, we can fill in two more "O"s before this block of two blacks indicating that these squares can only be black or background. The result would be: |