Reading the excerpt from the "Il Penseroso" (The Serious Man) by John Milton, 5the phrase "Prophetic strain" means an effort is required to be a visionary. The melancholic mood is glorified by the speaker as a means by which to "attain / To something like prophetic strain,".
The poem talks about the pleasures of the solitary man walking away during the evening. This poem is established in the mind of the speaker as he looks forward to visiting the places that he hopes a goddess named Melancholy will take him. The speaker will suffer a task that will outcome in experience, and because of it he will be in pressure, an effort spent that some forecasters had after getting some revelations.
Given that 'blunt' means 'not smart' and that 'mettle' means 'wit; intelligence', I believe that the statement that best paraphrases "What a blunt fellow is this grown to be! He was quick mettle when he went to school" is "This man, who was smart in school, has grown up to be unintelligent".
