Yes. The candles being burned for so many centuries within the cathedrals dirtied the glass, and made the buildings darker inside. There has been extensive cleaning of these in the 20th and 21st centuries. For example, the cathedral at Chartres had its glass removed and cleaned and replaced in the 20th century. It was a pretty complex and expensive job. I do not know if they did anything like that in centuries past. The thing is, a large amount of the soot is on the outside of cathedrals was deposited there in the 19th and 20th centuries from burning coal and other fossil fuels that caused a lot of pollution. That was not as much of an issue for the outside of cathedrals before that time.