the reason is as follows. I had recently removed the spark plugs after 50000 miles. ( as a routine, no trouble) In doing so, i realized that the color of one of the 8 spark plugs was very light. There were also some small white spots on it. However, i was not sure from which cylinder i had removed it. I wanted to see how the new spark plugs looked after 300 miles. I found this one black spark plug. This was the beginning of my confusion. But now i see that everything is fine. It is probably only one of the new plugs that is not quite right. I am replacing it now.