I think That blocking chimneys may have negative consequences, re damp and wall staining, though don`t know for sure.

I removed two double chimneys completely, and I put a gas cap on the one chimney left in use, and that chimney is not in regular use. With just the gas cap in place and without the open fire on, the room feels overall warmer than before when it had an open 10" bore chimney pot. confused2