R/O GI bleed.
If this is just one time, don't worry, it might be what you ate.
But if it has been long standing, these tarry black stools most likely mean you have a GI bleed, more likely upper, but can be anywhere. A full check of your GI tract is in order to locate the bleed. An endoscopy
would help to do this. If this persists, see your doctor to find and repair that GI bleed asap.