black poop

My 9 month old just had a black bowel movement, I called his dr & had to leave a message cause he’s always so busy! I asked my nurse sister and she said that urgent care will just take bowel sample but he only poops once a day. He hasn’t eaten anything other than spaghetti, formula, and his gerber puffs the past 2 days. I am worried cause I know what it could be but he isn’t acting in any pain and it’s very sudden. has this happened to anyone’s baby around this age and it ended up being fine??