If I cite a poem posted to a blog by someone who is not the poem’s author, do I need to write “posted by” and the blogger’s name in the “Other contributors” slot?
It depends on whether the person posting the poem is responsible for the blog as a whole.
If the blogger is responsible for the entire blog, list the author of the poem and the poem’s title. Then list the name of the blog as the title of the container. In the “Other contributors” slot, list the blog’s author preceded by a label, such as “created by” or “curated by,” that indicates the blogger is responsible for creating or curating the entire blog, not only for posting the poem. Then include the date of publication and the URL. The example below shows a works-cited-list entry for William Blake’s “Ah! Sun-flower,” a poem in the public domain that is reprinted on the blog Archetype. The author of the blog refers to herself as the blog’s “creator,” so you could choose “created by” to describe her role:
Blake, William. “Ah! Sun-flower.” Archetype, created by Michelle René Arch, 3 Aug. 2018, michellearch.wordpress.com.
If, however, you cite a poem posted to a blog by someone who is not responsible for the blog as a whole, and the site lists an editor, you might document the source as follows: list the author of the poem and the poem’s title. Then list the name of the person who posted the poem with the description “posted by” in the optional-element slot after the title, since the contributor does not play a role in the entire blog. Provide the name of the blog as the title of the container. In the “Other contributors” slot, list the name of the blog’s editor with the description “edited by.” Then give the publication details:
Rhymer, Sharon. “A Cup of Tea for Me.” Posted by Frank Thomas. Poetry Blog, edited by Lynn Smith, 20 Sept. 2018, poetryblogs.com.