Historical speculative fiction makes up a big chunk of the whole field, and when it's done well it's educational as well as entertaining. I certainly found Black Air to be both those things. Many others thought so too, since it was nominated for all the major awards, and won the World Fantasy Award for Best Novella in 1984.
I had never read speculative fiction about the Spanish Armada before. Robinson describes the conditions well, giving the inside dope on the rush job on some of the ships. We follow the teenage protagonist through his unusual spiritual development, where he learns the talent of seeing souls (as a flame above the head) from a friar on board. The talent can't save the Armada, but it does save him, in the end. Not too much speculation in it, really, but plenty of history. I recommend it.