We have discussed which blue is cobalt a few times, it is not made easy by many digital cameras having limitations when it comes to differentiating between blues.
My old camera could not tell the difference between the two blues in the following fish vase, the new camera makes a much better job of it. The central bubble is the typical pale blue that is more often found, the wings were formed by two dips in cobalt blue. The cobalt blue has a purple hue, when you have a piece with cobalt in your hand it makes it much easier to see the difference.
I suspect that fish vase on page 31 is not actually cobalt blue (if so, not the first error in the book).