I am so glad that you differentiated between the three quatrains of the poem and the final couplet, which of course changes the meaning of the poem completely. However, the celebration in the three quatrains seems to focus on the Harlem dancer of the title, and her ability to charm a group of prostitutes and drunkards with her skills. One of the most interesting examples of figurative language that is employed is the following quote:
To me she seemed a proudly-swaying palm
Grown lovelier for passing through a storm.
By comparing the dancer to the "proudly-swaying palm" that is only the more beautiful for having survived the storm, the speaker is making the point that this dancer, inspite of her insalubrious surroundings, is only made all the more beautiful because of the setting and the coarse characters that surround her. In fact, the various n'er-do-wells and public outcasts that watch her do not affect her at all, as she continues to sing and dance on, "gracefully and calm." The overall impact of her brilliant performance is that both boys and girls "Devoured her with their eager, passionate gaze." The three quatrains therefore serve as a celebration in praise of the woman who has managed to captivate her audience so completely.