Black spots on tempeh are typically not a cause for concern. In fact, they are often a sign that the fermentation process is working correctly. The black or grayish spots you may see on tempeh are usually clusters of the mold used in the fermentation process. These spots are perfectly normal and safe to eat. In fact, they can add a pleasant, slightly umami flavor to the tempeh. However, if you notice any mold that looks different from the characteristic black-ish discolouration or if the tempeh smells off or has an unusual texture, it's best to err on the side of caution and discard it. In summary, black spots on tempeh are typically a sign of the natural fermentation process and are safe to consume. Just be sure to use your judgment and discard tempeh that shows signs of spoilage or any mold that looks unusual or off-putting.