Hypoicons may be roughly divided according to the mode of Firstness of which they partake. Those which partake of simple qualities, or First Firstnesses, are images; those which represent the relations, mainly dyadic, or so regarded, of the parts of one thing by analogous relations in their own parts, are diagrams; those which represent the representative character of a representamen by representing a parallelism in something else, are metaphors. [CP 2.277]And where he goes on in other places to fill out what he meant by images and diagrams, this is pretty much it for metaphors. They run in parallel, a somewhat mystifying metaphor in its own right.
But Giambattista Vico says something interesting about metaphors that might help.
 By its nature, the human mind is indeterminate; hence, when man is sunk in ignorance, he makes himself the measure of the universe. [New Science, trans. by David Marsh, Element 1]In other words, we make a metaphor of one system we know, the "vehicle" of our own selves, with another system, the "tenor" in question. The tenor is seen as working in "parallel" with how things work within us. Although the vehicle is not always ourselves, I think it's fair to say, with Vico, that the initial approach to any unknown thing is metaphorical correlation with something we do know.
But Vico goes on to say, looking at these metaphors marbled into culture as "poetic truth":
 … Indeed, if we consider the question carefully, poetic truth is metaphysical truth; and any physical truth which does not conform to it must be judged false. [Ibid., 47]Human knowledge first turns to metaphor, to poetic truth, and then, only later, fills in the systemic details of the tenor diagrammatically. What's more, according to Vico, that physical truth must "conform to" the poetic truth. This seems questionable in the sense that any metaphorical understanding would surely be undone by different applications and delineations of rendering it diagrammatically. But then the metaphorical understanding could be expected to evolve as well. Perhaps it is a tandem unfolding of concepts in this way that Blumenberg is taking advantage of when he traces the historical evolution of concepts of like "progress" or "truth" metaphorically.