Texas Angel by Judith Pella

Elise Toussaint Hearne thought her devastating secret was safe after all, her dark hair and complexion could mean she had French ancestry. But that did little to save her when it was discovered that her deceased mother was a runaway quadroon. Disowned and shamed by the Hearne family, Elise and her baby find themselves on their way to the wild frontier of Texas as slave. When Elise inadvertently meets Benjamin Sinclair, a fiery evangelist determined to convert the lawless Texans, their lives become intertwined in the midst of tragedy shattering their fragile existence and forcing them to discover the faith that sustains.