I've seen girls in about even numbers wearing blouses and a button down, as well as even numbers of pants and skirts. The tops also don't have to be white. Blouses shouldn't show cleavage or have anything distracting on it. I don't think either the blouse or the button up is more appropriate than the other. Do whatever is comfortable and makes you feel confident. I personally feel more comfortable and confident in a pastel-colored blouse because it softens the pantsuit look up, but it's up to you what you prefer.