After an endoscopic exam to the stomach doctor send to biopsy some dark tissue. He said it s maybe cause smoking! Should I be worried?

Smoking and cancers. Unfortunately, even with the best equipment, there is often uncertainty about what is being seen through an endoscope. As smoking can cause cancers in the esophagus and the stomach, anything other than normal will raise concerns and possibly lead to a possible biopsy. Yes, there is a cause for concern, but fortunately more biospies are negative than positive.
As abovde. It is best to discuss this with u provider.