I'll go a little a deeper into African-American slavery and suicide, specifically in the United States. It was always a choice for African-Americans do use suicide as a form of resistance. This link provides several excerpts from a variety of narratives that discuss the motivations and causes among African-American slaves. In these excerpts, we see African-Americans use suicide to exhibit control in their lives. Martin Jackson's mother committed suicide apparently as a symptom of a mental health issue. Depression and anxiety were likely common maladies among African-American slaves. Slaves may also have used suicide as a last act of desperation. Slaves who had escaped, like in William Henry Towns' narrative, may have chosen death as opposed to being returned to their masters.
Linda Kay Kneeland dedicated her thesis towards defining the suicide rates among African-American slaves in the antebellum South. Statistically, she claims that suicide was not a common occurrence. Kneeland makes a valuable observation that historically African-American suicide rates have been lower than rates among European-Americans (p.6). Kneeland attributes the role of religion and the promise of freedom to be the primary deterrent of suicide among slaves (p.108). The period of time where suicide was most prevalent among African-American slaves was the beginning of slavery in the United States. Some imported Africans knew of life before enslavement and would choose death over enslavement. Kneeland also notes that suicide was disproportionately represented in fictional slave narratives (p.56).
Kneeland's work is fascinating! However, there are many limitations towards any statistical analysis of slavery in the US. As an economic system, I do not doubt that recording the suicide or even death of a slave was a common practice on plantations. However to concentrate that data into a single study would be almost impossible. Since there was no unified system of data collection among slave owners, what might be ruled an "accidental death" on plantation could be defined as a suicide on another.
Sources
http://nationalhumanitiescenter.org/pds/maai/emancipation/text2/suicide.pdf