In 1761, Susanna and John Wheatley purchased a frail, sickly child who was around seven or eight years old from the captain of a slave ship, who assumed the girl was about to die and wanted to realize some profit before she did so. When the Wheatleys discovered the girl's precocity and intelligence, they taught her to read and write and then gave her various classics of Latin and British literature to read, including those of Virgil, Ovid, Milton and Pope. When she started to write poetry, the Wheatleys encouraged her endeavors and even found a publisher for her in England.
Given this background, it is difficult to see how Phillis Wheatley's poems could have been composed in anything other than the standardized American manner, or, indeed, in the standardized British manner, since most of her models were British. It seems unlikely that the Wheatleys ever explicitly urged her to follow these models, but this is primarily because they would not have had to do so. The young poet absorbed the influences with which she was provided. The idea that she should write for Africans, or for African-American slaves, probably never occurred to her. If it did, we have no record of it.
The question of Wheatley's message is a difficult one. She often writes panegyrics or epitaphs in which the main message relates to the qualities of her subject. Her broader message is generally one of conventional Christianity. Several times she makes the argument (which is profoundly uncomfortable for modern readers) that she was saved by her slavery, since it brought her to Christianity. She makes this point in what may be her earliest poem, "To the University of Cambridge, in New England." The message is one of embracing Christianity and avoiding sin, and Wheatley thanks God for delivering her from "the land of errors" and bringing her to America. If she has a more radical message which has been lost, then it remains very thoroughly lost.