Who could resist a blue cactus?! I certainly can’t. The blue coloring is a kind of illusion: the cactus’s skin is actually green, but as a form of sunscreen it produces a whitish waxy coating that, on top of the green skin, looks blue. You can see the same waxy coating on other cacti, making them look blue to some extent, but this is more brilliant than the blue you would find on the typical, say, Parodia magnifica. (This isn’t specific to cacti—a similar coating makes blue spruce needles look blue.)
I assume this is a young example of the commonly cultivated P. pachycladus, but I’m not sure. In a container this plant will never reach its full potential, but I hope to keep it growing well enough to produce some flowers and arms, eventually. It likes full sun and a consistent feeding of fertilizer at a low concentration, at every other watering (I grow in almost pure pumice; this may not hold true for plants grown in soil).