Throughout the poem, the speaker encourages the eponymous "blue girls" to celebrate and enjoy their beauty and vitality while they last. In the very first line of the poem, the speaker evokes the image of the girls "Twirling [their] blue skirts," and later, he describes their "chattering." The dynamic verbs twirling and chattering create an impression of the girls' movement and energy.
In the second stanza, the speaker uses a simile to compare the girls to bluebirds. He tells the girls that they should no more think "of what will come to pass / Than bluebirds" do. By "what will come to pass," the speaker means the old age and infirmity that will inevitably come to each of the young girls. He is telling them, in other words, to enjoy and revel in their youth while they can. A bluebird does not think about the passing of time or the loss of youth, but exists happily in the moment, from one moment to the next.
The speaker also uses repetition throughout the poem to emphasize his message that the girls should enjoy their youth while they have it. He repeats the word blue, for example, four times in the poem. The word blue is a vibrant primary color, and on the one hand, it connotes the energy of youth. On the other hand, the word blue can also be used as a synonym for sad. If somebody is described as "feeling blue," it means that they are feeling sad. The speaker might repeat the word blue, therefore, to also imply that the girls are beginning to feel sad because they are beginning to think about "what will come to pass." They are beginning to appreciate that one day, they will no longer possess the youth, beauty, and vitality that they currently enjoy.