Sleeving was pioneered by Chris Ashthorpe. There is some information here http://worcestergunsmith.co.uk/history.html
In the earlier days of sleeving, the joint was always visible, I think because the new tubes were soldered in place?. I believe improvements in welding techniques allowed TIG welding to be used without the heat damage that would have been incurred with earlier processes. Note that many sleeved guns had damascus breechings, and steel tubes, so that the blacking had to be even across two different materials, difficult to achieve invisibly. The best sleeving nowadays is very good indeed at hiding the join.