The poem does not state exactly who brought the dark girl across the sea because the dark girl is the Negro Mother of the poem, and of course, in the poem, the Negro Mother is a general term for all African American women who were enslaved for over three hundred years in the southern states of America.
In the poem, the Negro Mother says that she was the "dark girl who crossed the red sea." She doesn't expand on that particular part of her journey any more than that, but does say it was the beginning of a longer journey that lead to the freedom of the southern slaves. Throughout she says she was "beaten and mistreated" and had her "children sold away." Yet somehow she managed to survive until one day she was free.
The point of Langston Hughes's poem is to state how important the Negro Mothers were to African Americans in their fight for freedom. They were, as he says, the "seed of the coming Free" and neither they nor the hardships they endured should ever be forgotten. Moreover, their spirit remains in the current fight for the rights of the American Africans. The Negro Mother herself says that she hopes that her example can impel them forward and help them break new ground in their march towards equality.
As the negro mother states at the end of the poem:
For I will be with you till no white brother
Dares keep down the children of the Negro Mother.