Stevens' metaphysician in the dark is pretty darn complicated. But, at its heart, the image suggests that a modern poem is like a metaphysician—a type of spiritual philosopher who asks basic questions about what reality is and why human life exists.
The fact that the metaphysician-poem does its job "in the dark" suggests that modern poets need to think with their hearts instead of their eyes and ears. It also means that modern poetry has to respond to the spiritual darkness of the modern world with a message of hope.
Finally, the modern poet is in the dark because modern poetry, for Stevens, has to play out the process of searching for something. Modern poetry has to be about the human mind looking for some deeper meaning in life, which makes the image of a metaphysician wandering in the dark a pretty good one for conveying Stevens' point.
Line 20 is the only time the image appears in the poem. In the next few lines, though, Steven hands the metaphysician a guitar and gets him/her to pluck a string on it. For more info about the significance about this guitar, see the entry in this section for "Twanging Guitar."