In this poem, the switch from merely literal to symbolic seems to take place in the first line of the third stanza, when Dickinson shifts from discussing "Dark" (in the first two stanzas) to "Darknesses — / Those Evenings of the Brain." The fact that she makes darkness plural and refers to those darknesses as being something within us is a significant clue as to the meaning she ascribes to darkness. For her, in this third stanza, the darknesses exist when there is "not a Moon disclose a sign — / Or Star — come out — within." A moon or star within us would seem to signify a guide of some kind, something we can look to and follow when life feels unpredictable and inconstant, when we feel unsteady and insecure. Thus, darkness refers to this sense of insecurity or doubt.
Those who are "brave" enough to keep moving forward, despite the darkness of their vulnerability, sometimes do bump into things, so to speak: they make missteps as a result of their inability to see (or know) what's ahead of them. But, eventually, their sight adjusts to the lack of light (or the lack of a goal or an idea, something that would guide them with less uncertainty), and they manage to walk just about straight again, or, literally, to move forward with less hesitation. Thus, when we bravely face our insecurity or vulnerability, we learn to function, to thrive, in spite of it.