If you happen to accidentally eat one or two bad blackberries that had mold on them then nothing will usually happen to you.
However sometimes you might get a stomach ache, diarrhea or vomiting.
Usually it's nothing to worry about and the stomach ache, diarrhea and vomiting will usually go away on it's own within a day or two.
If you notice mold on the blackberries then throw them out just to prevent sickness.