This is a rare view of The Bell unless you live on The Paragon. The blackened wall is the rear of the original 17th century coaching inn and the other roofs are Victorian add-ons.You can see a bit of it by standing on the steps leading to the back bar, of course. You can also see some of the original little casement windows which have been blocked in years ago.
The brown/black colour of the stonework was mainly caused by Victorian/Edwardian coal smoke from domestic fires. Up until the 1970s most of Bath was that colour. The huge retaining wall below The Paragon in Walcot Street was never cleaned. Underneath that soot is pale stone.
If you look at the joints of the Bell's rear wall, you can see whitish lines where the putty-lime mortar has crumbled away a bit. The coal soot contains a lot of sulphur which turns very acidic over time, causing damage to the stone at its weakest points.
It would be good if the Board would aside a bit of money to clean and repoint this original back wall at some point soon, before it gets any worse. I know that it is not very visible from the garden, but it is a rare bit of stonework hiding away amongst all the Georgian stuff, and Bath has very few 17th century buildings left standing.