Without looking at the game, i will assume that the last move filled the bottom row, making it dissappear and everything else drop down. The blue square is to indicate the last move was in that column. It is very possible to have several moves in the same square (in the bottom row only) because of this rule. Hope that helps!
Kevin
I'm playing this game with Dawn and on the game notations it has both of us moving on B1 It showed up as an empty blue square on my turn. Tried to play on top of it and couldn't. It let me move on B1 also
When the board fills up, is the game a draw, or is it a win for the first player? If it is a draw, the second player can simply mirror what the first player does (same thing on other side of board) and never get Four in a row (only the first player can). If it is a win for the first player, it should be specified in the rules. Thanks!