“without attaint o'erlook”
'Attaint' means dishonor, moral taint, or imputation of wrongdoing. To read other poets' dedications 'without attaint' is to do so without incurring any stain on one's reputation — no disloyalty, no shame. The verb 'o'erlook' here means to look over or read through, not to overlook in the sense of miss.
“fair in knowledge as in hue”
'Hue' means complexion or outward appearance — the color and look of a person's face. The line compliments the youth doubly: he is as perceptive a reader ('fair in knowledge') as he is beautiful to look at. The pairing of inner discernment with outer beauty is a standard Renaissance conceit for the ideal beloved.
“fresher stamp of the time-bettering days”
'Stamp' is the impression left by a seal or coin-die — something that makes a mark of recognized authority or value. A 'fresher stamp' is a newer, more fashionable style of verse, as if the rival poets offer a newly minted coin while the speaker's praise is old currency. 'Time-bettering' means improving with the age, up-to-date.
“truly sympathiz'd In true plain words”
'Sympathiz'd' means depicted in exact correspondence with its subject, faithfully represented — not the modern sense of feeling sorry for someone. The speaker claims he rendered the youth truly because his words matched the reality, while the rivals' rhetoric distorts and inflates. 'True plain words' is a pointed phrase: plain means unadorned, honest, undisguised.
historical The sonnet form- TLN 1147rhetorical device
“their gross painting might be better us'd Where cheeks need blood”
The couplet is the sonnet's volta — the closing turn that clinches the argument with a concrete image: 'painting' means both cosmetics (rouge applied to pale cheeks) and elaborate verbal ornamentation. 'Gross' means thick, heavy, coarse. The rivals' overdone rhetorical embellishment would be useful on a face that actually lacks color; applied to genuine beauty, it is 'abus'd' (misused, wasted, corrupted).
historical The blazon (and the anti-blazon)