Ah, metapoetry. And excellently done.

I think anyone who's ever put pen to paper can probably relate to this easily. The contrast between the first two stanzas where everything is wild and creative, so magical, and then the third where everything has gone wrong (for the poet, not the poem) is just as incredibly striking as we all know it to be in real life. It's that contrast, I think, which makes it so perfectly resemble the way we feel re-reading things that we were so recently aflame to write.

A few notes on phrases that stood out to me:

This youth haunted by language, his head
invaded by phrases that some disembodied tongue
might have shaped.

Our introduction to the wordsmith, and what an introduction it is. That phrase "haunted by language," feels very apt as a description of the pre-writing process, where maybe a phrase or two have circled your head for some time moaning and clanking their chains to get your attention so you'll write them down. The whole of the sentence gives us our character as a conduit for the poetry, more than a writer. "invaded by phrases" and "disembodied tongue" carry this idea on beautifully, whilst fitting aptly with the preternatural nature of haunting established right at first.

He scribbles, hoping that his hearing will
be keen enough, that he will know enough to transcribe
the plainsong of his passions and fears.

Choosing "scribbles" over the many other words for writing was a good choice, here. There's a frantic edge to the second stanza that builds on the magic of the first. This one is the stanza that deals with the act of creation itself. As I've said before, this is eminently relatable, although perhaps less so, as (for me at least) this is only the way it feels at the very best of times. Again we have the image here of the poet not so much writing as just providing an escape for the poetry. Transcribing, as you put it. "plainsong" keeps that wildness and fervour to the piece, and then ending the sentence on "passions and fears." really reinforces the emotional nature of the work.

Later, he awakens. Day proves to be unhelpful.
He scans the paged words as they lay inert, paralysed.

As I said, the third stanza is where it all changes, and you've wasted no time in making this very clear. After the purely narrative first sentence we have this: "Day proves to be unhelpful." Rather witty in its dry, deadpan way that really slams home the deflation of the mood. "paged words" is an interesting phrase. I'm not sure if page is usually a verb, but it works well here. Reminds me of "floored" with all those connotations of being actively knocked down, in this case by the act of writing itself.

His text is obdurate, lumpen and solid
now his fiery words.

I enjoyed the intervening image of the blacksmith but have decided to jump right to the payoff for that set up. The separation of the line into two has worked wonders to keep that second last line stay cold and dissappointed. The words you've picked there are such perfect counterpoint to the rest of the poem. Obdurate is a word I love anywhere, and then "lumpen and solid" at once evocative of the blacksmith image of cooled metal and so directly in contrast with their previous flowing, mercurial nature.

The use of "fiery" in the final line serves to reinforce the contrast brilliantly by linking it back to that whirlwind second stanza.

I like this whole poem a lot, Alec.

I'm new to the critique thing, I hope I've done it ok.