It's only when a cheesemaker pricks those wheels all over with long steel needles that the blue mold will start to proliferate. Those needle pricks introduce oxygen to the inside of the wheels. Without the oxygen, blue mold can't grow. It's the key to blue mold survival. Once oxygen is introduced, blue mold begins to travel outwards, closer to the rind, ripening the cheese as it migrates outwards.
Blue cheese is the only style of cheese that ripens from the inside-out as opposed to the outside-in. (Reason #2 that makes blues unique.)Often, you can actually see the distinct paths of blue, like long striated lines, like at the top of the the slice in the picture at left. Or you'll see small circular holes where the needle went into the wheel and where blue mold began its growth, like in the picture of Roquefort at the top of the post.
And sometimes, if it's been freshly cut off a wheel, a new slice of blue cheese will become bluer and bluer before your very eyes, a trick that's sure to impress if you can time it right.
Related: The Cheese Monger: Bleu de Gex