Vomited with blood. er doctor said, since it's a one time event and i don't have any other symptoms, further treatment isn't necessary. basic blood work was fine. he said something how this blood could have occurred, which is nothing to worry about?

Vomiting blood is disconcerting & it is good that you had an evaluation by a physician. If it was a tiny amount of fresh, bright red blood with no further vomiting,abdominal pain or black tarry stools, I would agree that it wasn't a serious issue. Probably only a tiny vessel burst.and the bleeding in the stomach or esophagus quickly subsided.

Follow up is always needed, ideally with your primary care physician that can further coordinate in case needed, specifically if symptoms return or having new, unexplained symptoms.