The charring of wood as a method of preservation is nothing new and was known to European and Mediterranean builders as well as Asians. I have for years charred the bases of deck posts and such as a rot preventative. The practice is mentioned in John Fitchen's classic book Building Construction Before Mechanization. Some of the samples of this shown above are a bit crude. The most beautiful finishes of this type are rather labor intensive as some of the char can be brushed back off to some extent to reveal wood grain. The planter shown below has been coated on the interior with linseed oil, blackjack and turpentine and then set alight with a roofer's torch. The finish is cooked into the wood producing a carbonized insect and decay resistant surface. It gets filled with soil and so the char will be seen only at the edges.

The fading black I have seen has been on manufactured plywood type cladding. It does not look good as looks uneven and patchy. When building we changed from the cheaper cladding to solid recycled hardwood shiplap. So glad we did, as it is now greying off beautifully.

Got my answer - non combustible material such as, natural stone - marble, granite or concrete etc.

I wonder how much time you might have to spend cleaning the roof behind the parapet, perhaps the rain will take care of that, leaves?. If you were going to use these spaces as terrace you would need to adjust the height of the parapet or add a railing, if not the height of the parapet could be reduced