Painting poor quality slate roofs with bitumen is sometimes called ‘Turnerising’. I don’t know the origins of the name or where it started.
When slate nears the end of its useful life it starts to delaminate. A quick and cheap way of trying to keep the roof watertight was to paint it in liquid bitumen, It was a bodge repair and should never be done on a Listed Building – or indeed any building. Rain can get in under the cracked bitumen and this accelerates the damage to the already damaged slate.
Removal of the bitumen is not a viable option. The whole roof will normally need to be stripped and renewed.