I had a one off hard, dark stool. i'm not sure if it was black but should i be concerned as it was a once off? i'm 21 and in otherwise good health?

Altered color stool: Color of stool varies with foods, speed of emptying of the GI tract, medications ; supplements ingested, gut flora (may vary per individuall), additives (like blood), subtractions (like absent bile), infection/exposures, inflammation, ; other factors. Probably okay if a single occurrence but persistent dark stool warrants exam ; treatment. Avoid nsaids, aspirin, etoh 4 now to minimize bleed risk.