The Blue Back Speller was the most-used tool for enslaved literacy.

Evidence

In Heather Andrea Williams’s book Self-Taught: African American Education in Slavery and Freedom, she writes: “The tool that African Americans used most frequently to decode written English was Noah Webster’s Elementary Spelling Book, popularly called the Blue Back Speller
it was this little book that Frederick Douglass and countless other enslaved people used in the first steps toward literacy. And when slavery ended, adults and children, many of whom could not attend school, got a hold of the Speller and slowly taught themselves to read. The Speller accrued emotional significance as it helped individuals to decipher written language. 87-year-old John Walton expressed his sentimental attachment when he told an interviewer; ‘I learn to read and write just since freedom. Us used Webster’s old Blue Back Speller and I has one in the house to this day-I wouldn’t take anything for it.’ ” The Papa Renty stories I heard growing up said he studied well into the night, every night, until he understood and mastered the entire speller. — Tamara Lanier, From These Roots, 220