In a poem running nearly 150 lines, it would be weird if there weren't any sound effects to keep our ears on their toes (if you can picture that). Luckily, "The Blessed Damozel" is no exception. It's got a ton of great sonic techniques to lend its lines an extra punch.
Take alliteration, for example. This poem lobs lots of linked language into our laps. We start with repeating D words in line 3: "Her eyes were deeper than the depth." From there, we get S words packing lines like "She scarce could see the sun" (30) and "The stars sand in their spheres" (54). Rossetti's not done there, though. Harken to all the H words in "Herself shall bring us, hand in hand,/ To Him round whom all souls" (121-122).
In addition to its alliterative moments, the poem offers up some consonance to tickle our ear buds. We have lots of L sounds in lines like "And the lilies lay as if asleep/ Along her bended arm" (47-48). And F sounds abound in lines such as "Was like a little feather/ Fluttering far down the gulf" (56-57).
Finally, we even get some fancy internal rhyme in line 136: "The light thrill'd towards her, fill'd." The key here—as with all of these types of sound—is repetition. These examples (and there are plenty more to find on your own) create mini-echoes throughout the poem to emphasize lines, phrases, or ideas in a subtle way.
They do more than that, though. In a poem that deals with separation, eternity, and all the waiting that goes along with it, repetition—even on the level of sound—is an important idea. Each day seems like the next for our separated lovers, a monotonous march until they can be together. And all the repeated sounds of the poem mirror that sense of endlessly repeating days and days and days and…you get the picture.