Well all the Advent ships have names that mean something in English, (Rapture/Halycon etc.), and while at first I didn't think the Cyhyraeth followed that pattern, I quick look on wikipedia informs me its some Welsh banshee like creature, which while a bit different from the other Advent ships certainly fits them. Perhaps this slight change in nomenclature indicates the Cyhyraeth is a Rebel ship?
Also from wikipedia, Nigredo was once a alchemy concept for decay, but psychology has since taken over the term to mean the process of when an individual confronts the darkness within themselves. Which does follow quite well with the previous ship names.
A Cherubim is of course the little guy below, from Judeo-Christian mythology and fits with the religious based meanings of previous advent ships, and seems to be the basis for the odd ship shape. So I suppose it does make a little sense to call it that instead of a battleship or whatever.
Though the devs might have had this sort of version more in mind.