All I know is a door into the dark.
Outside, old axles and iron hoops rusting;
Inside, the hammered anvil's short-pitched ring,
The unpredictable fantail of sparks
The first  quatrain  introduces the themes. “Dark” suggests that the area into which the door leads is mysterious or forbidden to the speaker. 
 The line presents a contrast between the dark, the older time period or the blacksmith’s profession, and the outside, modern world, in which his work is needed less and is almost invisible. 
 It is also a  metaphor , an oblique reference to more mysterious depths. The blacksmith’s workshop is also a place of alchemical significance for refining the soul.
The Forge