The “He” in this poem is ambiguous, it could be a public speaker, a preacher, God, her own father, or even Zeus or some male aspect of thundering nature, like a storm. Either way it’s an authority figure of some sort that begins with a fumbling (which seems almost careless), then leads to a sustained “Blow” (which feels violent), and ends with “still” (and firm and pause / “Paws”).
As in “He put the Belt around my life“, there is a violence embedded in the “He” character. In that poem “He” has placed a belt on her which could also be a sort of infliction and not necessarily the gift of poetry / art that she describes. To “Belt” someone is to hit them, and thus in this poem “He fumbles” which, though not quite as directly violent, still feels like a violation, like a young man whose greedy fingers won’t let go of a young woman.
What the “He” really reminds me of is a storm, such as a gale, or Nor’easter, or hurricane that would have been familiar to Emily in New England. These are violent storms that start off slowly (“fumbles”) then they pick up in intensity and until they deal “imperial Thunderbolt[s]”. Yet this is nature’s music, nature’s violent art that comes from the same clouds she describes as being a member of in “He put the Belt around my life“. Thus perhaps she is saying that creating art is like a storm, a violent confluence of emotions and energy whipped up and mixed up like an “etherial Blow” that destroys everything in its wake and leaves a devastating silence behind.
What’s interesting is that Emily usually associates nature with the female pronoun, but here nature is associated with the male pronoun, so perhaps she sees nature as having a dual personality, it has male and female qualities – which makes sense since life on earth is typically divided up into male and female – but this “He” has a violence about it that is unlike the nature which grows flowers and sees after gardens. Here it destroys, not builds, yet even in destruction there is renewal in the silence after the storm. So perhaps this is what the creative process feels like for her, in that inspiration comes on like an angry male God, throws everything in her mind into a whirlwind which she must weather, and only after she’s ridden it out can there be peace.
Thus could this be who the mysterious Master is? A male-centric force of violent inspiration? A storm of the imagination?