There is only one source for Eliza's maiden name: the death certificate of her son Gordon Cephus Black, which gives her maiden name as Liza Ann Black. It is possible this was her married name because her maiden name was unknown, but it is equally possible that her maiden name was Black.
FamilySearch Family Tree places both William Black and his wife Eliza Ann as the children of Samuel and Mary (Norton) Black. While this is possible, it is not the only possibility. Regardless, if James (born 1839) was their son, as claimed by FamilySearch Family Tree, both William and Eliza Ann appear to have been living with Samuel as early as 1840 (after Samuel's wife Mary presumably died). The head counts for the Samuel Black household in the 1830 census could accommodate both or either of William and Eliza, making it currently impossible to determine which was the child of Samuel and Mary (Norton) Black (assuming both weren't). Given the fact that William's surname is certain, while Eliza's maiden name is less so (only one source, which could be incorrect), it seems reasonable to place William in the Samuel and Mary Black family.
Note: FamilySearch Family Tree has several unsourced exact birth dates for people in this family. It also cites "legacy NFS source" and implies that there are "family records" for some of this information, including Eliza Ann's maiden name.
