The "low divorce rate" is just a trick of the numbers. In truth, it is low sample size, advanced marrying age, corresponding wealth which account for the low marriage rate in BW/WM couples. These variables lower the divorce rate for every couple, it's just that all other couples are more evenly distributed across the population, whereas the BW/WM is not. Older people and richer people get divorced less in general (younger-married and poorer people get divorced more).
Let me explain: in normal society, blacks and whites marry at far younger ages to their own groups respectively, which pushes down the average age and the average wealth (as well as education) down relative to the BW/WM. The normal black and white marriage population is "representative" of the groups, while far more rare (like BW/WM) is not. So the "low divorce rate" for BW/WM couples is merely a trick of the numbers. It is most likely a function of 1) low sample size and 2) variables like older average marrying age, wealth, education, etc (which only matter b/c the number is so low in the first place). If you control for older marrying age, wealth and education for white couples (for example), the divorce rate for wealthier white couples (that married later) undoubtedly falls, which makes the case. As such the BW/WM marriage rate isn't "real".