A casement high and triple-arch'd there was, All garlanded with carven imag'ries Of fruits and flowers, and bunches of knot-grass, And diamonded with panes of quaint device, Innumberable of stains and splendid dyes, As are the tiger-moth's deep-damask'd wings; And in the midst, 'mong thousand heraldries, And twilight saints, and dim emblazonings, A shielded scutcheon blushed with blood of queens and kings.
This entire stanza basically just describes a stained-glass window. Because Keats's motto in this poem is "Why use one word when I could use 60?" This stanza is also weird because in stanza 23 you thought you were really getting somewhere. At last, Madeline arrives, and she's... panting. References to super-dramatic myths are made, aaaand then… "Hey, would you look at that window?"
To be fair, it's a pretty window, decorated with pretty fruits, flowers, and dyes that is compared in a simile to a moth's patterned ("damask'd") wings. That said, it's not all pretty description: this window also features a "thousand heraldries" surrounding "a shielded scutcheon blush'd with blood of queens and kings."
Okay, so, what does that mean, and why does it matter? What Keats is basically describing is the shield (that's the "scutcheon") in the middle of a coat of arms that represents Madeline's family.
Wondering why Keats takes a commercial break to talk about this? Go check out "Symbols: Scutcheon" for more some ideas, but come right back.