Follow up question: does anyone have photographs of it when it was black?

It was cleaned in the late 90s, as was half of the UK, it seemed at the time. Everywhere you went during the years leading up to the Millennium, many of the cathedrals and other historic buildings were covered in scaffolding having the Victorian coal dust and grime cleaned off.

Many of the blocks were replaced on the Abbey, as you can see, since they're lighter in color than even the cleaned ones. I spoke to one of the masons repairing a spire at the top as he was doing the final carving on a piece with a mallet and chisel. He was from Malta and his whole family was in the stone mason business and had been for generations. Apparently it's still a thriving skill there.

I can't speak for before and after, and I don't know about the public's response, but to my eyes it made the city look fantastic and there was a sense of "cleanliness" and optimism going into the Millennium, partly due to the growing economy but also helped by the sense that London was returning to a former, rightful, and hitherto unknown, glory.

Edit: Wow! So many upvotes! Thank you! That makes this my most upvoted comment ever. :)

I am curious how they cleaned them. I am sure pressure washers would work but I would worry about eroding the stone of some of the old structures.

Does anyone have any info on how they did it?

Soot from coal used in household fireplaces blackened many buildings.

Presumably when you refer to "london fog" it's some sort of euphemism for that?

A restoration effort in the 90s and 2000s saw many buildings undergo cleaning to remove the blackening. The restoration of Westminster Abbey would have been seen in the context of a larger scale effort which saw many historic buildings cleaned.

There are cities in which this restoration effort has not taken place to the same extent, for example Edinburgh where many buildings are still black from soot.