Symbol Analysis
The scutcheon, or as well like to call it: that part where Keats takes a nine-line time-out to talk about a window. Just when you think the sparks are about to fly between Porphyro and Madeline in stanza 24, Keats starts talking about a stained-glass window in Madeline's bedroom, zooming in on a coat of arms painted in the middle of it. We promise, though, Keats isn't just doing this to harsh your flow.
And in the midst, 'mong thousand heraldries,
And twilight saints, and dim emblazonings,
A shielded scutcheon blushed with blood of queens and kings. (214-16)
These lines probably reminded you of the "sculptur'd dead" (14) that Keats talked about in this chapel, which felt like a different world; the chapel is made out to be the place of chilling penance and servitude—to God and to your family. So what's it doing in the bedroom scene, right when we feel like we're getting to the good part?
The primary conflict of the poem's plot is that Madeline's family has this age-old beef with Porphyro's family. So, behind all of the action there's this heavy, unavoidable backdrop of dynastic oppression, crushing down on these two kids who're nuts about each other. It makes sense, then, that Keats brings that theme back into the fore at the moment where the two are finally united.
That said, the scutcheon breaks the poetic rules that Keats has thus far established: the language of the chapel was universally cold and dead. Our scutcheon, even though it's a static piece of art, is described as "blush[ing] with the blood of queens and kings." Blushing and blood are words that come straight out of the Young Lovers' Dictionary—Keats goes to great lengths to convince you that these two are warm, living, physical, passionate people. In a poem that relies heavily on stark binary contrasts (see "Wordplay: Contrasts") (hot-cold, awake-asleep, filial servitude-romance), the "blushing" scutcheon becomes a symbol that brings together the two most harshly opposed dramatic forces in the poem: familial loyalty and young love.