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
(Cacher) Si vous souhaitez saluer quelqu'un dans sa langue natale, essayez notre Dictionnaire du Joueur, par le lien "au sujet des langues" sous les drapeaux. (pauloaguia) (Montrer toutes les astuces)