Out of all the basic volleyball skills he learned in the past, blocking was probably what Riku lacked the most. It wasn't like he was bad at it, but he always found an excuse to not put all of his effort into doing that. From an excuse that he couldn't set properly if he joined the blocker, couldn't synchronize with the blocker's movement, to even a ridiculous one like no one thought being a blocker was cool, he had already thrown all of that as an excuse to get excluded from getting dragged into the front side.
However, once again, that didn't mean he was bad at it.