I am thinking Guangxu or Tongzhi maybe? Is the blue the right color?
Right on spot.
The bottom looks 18th century, the mark is called the endless not.
You are right about the bottom Stan. However, the decoration and mark are 19th century.
The bat underneath the rim is late Qing dynasty.
do you think the decoration could have been added later?
Sure. It is a bit unusual that late, but the plates of the Jiaqing reign, for example, are frequently of a type attributed to the Kangxi-Qianlong period. It is thought that the fired blanks in storage did last for decades after the Qianlong reign.
Thanks for the good information.