I have read that many thread manufacturers do not specifically dye black thread. They over dye colours that have not been selling well ie peach. As a result of being processed more than once, black thread may not be a strong as other colours. You could also have a burr on your needle or in your bobbin case that is breaking your thread. If it's only breaking black thread, I'd be suspicious that it is inferior quality that is causing your problems. I find YKK thread really stong.