Blanche of Spain is King John's niece. Like most women of her time, especially those born into royal families, Blanche is basically a pawn in political games that she can't control. In this case, she gets married off to Louis, the Dauphin of France, in order to cement an alliance between her uncle, King John, and Louis's father, King Philip of France.
But just because she doesn't have any control over her destiny doesn't mean she can't think for herself. Blanche clearly has a powerful mind—which she expresses with her biting tongue. Her words when asked if she would marry Louis are a masterpiece of understatement and sarcasm, saying that she'll do whatever her uncle wants, and that hey, she doesn't see anything in Louis worth hating. Whoa, hold back on the big compliments, Blanche.
Later, when it looks like her new husband and his father are going to break their new alliance with King John, Blanche speaks up against the move. Once it becomes clear that the two kings will totally go back to war with each other, Blanche speaks movingly about how she is now caught between a rock and a hard place. This is the last time we hear from her during the play.