Superstition is a recurring motif in The Adventures of Huckleberry Finn. It also acts as a catalyst, propelling the characters to make choices and take on tasks that they otherwise would not have considered.
Another motif is that of appearances. Not all is as it seems. This is certainly so in the case of the Duke and the King. Even Huck disguises himself. But, on a deeper level, Jim is not what he seems. On the surface and to those not acquainted with Jim, he is just a slave. Huck comes to know him as much, much more than that.