Black's is the definitive work on Scottish surnames, despite its age (it was first published in 1946 by New York Public Library). You can rely on Black to have covered almost every source publication before that year, but works published since then may very well have material (and so names) unknown to Black. In addition, Black had to depend on existing indexes within books, and many of these in Victorian times omitted the "less important" people. As a result, surname examples earlier than that shown by Black are possible (my own copy has a number of inserted notes). Nevertheless, you would find it impossible to find a better book on the subject.