I love the comment on sun-bleached black paint on the upper floors, but it's completely wrong. These black bricks are not black because they've been painted - they're black with coal soot from before burning coal was banned in the 1950s. If they're less black on the upper floors, I guess it's because 60 years of rain eventually has some effect.
Until relatively recently, most of the tourist postcards of London you could buy in the tat shops in town had old photos where the buildings were all blackened. I'm not sure when they were cleaned, some time in the late 70s or early 80s perhaps. A dramatic difference!
