This approach is consistent with what I have read about how blending is done, notably in The Earnest Drinker, Oscar Mendelsohn, 1946, written by an Australian professional chemist who combined a technical perspective with a connoisseur's interest in drink. He was describing what distilleries normally do and the approach is almost exactly the same. Mendelsohn referred also to using very small amounts of Campbeltown whiskies, with or as an alternative to Islay whiskies for the pungent element. At the time, there were more Campbeltowns than today and they seemed of a more pungent character than surviving examples.
I know that some blended Scotches use spirit caramel. In my view, this is a kind of blending agent, if enough of it is used anyway, because some scotch whisky seems to disclose a taste attributable to this element.
I do not know if U.K. law allows blending agents to be added in the sense of sherry or brandy or fruited concentrates of some kind.
While by the 1940's it had become settled that approximately half of a blend (sometimes more) was grain whisky and a good part of the rest Lowlands, which is relatively bland due to a more thorough distillation method and lack of peating, earlier on higher quantities of malts were sometimes used, for luxury blends. (And that still may be true for certain brands today). I have experimented by simply adding more malt to a given blend, to lower the grain whisky component. Oddly perhaps, I have not found a blending agent necessary, I think this is because what I added often included sherried whiskies which is not dissimilar to adding sherry as a blending agent. But also, if a blend is well made, you don't need a blending agent, I think where still used they are often added mostly to ensure a standard color.
Here is a modern discussion of whisky blending written from a whisky technology standpoint:http://books.google.com/books?id=Iukpfh ... ky&f=false
The writer refers to use of spirit caramel by some blenders but does not mention blending agents as such for scotch whisky production.
I have my favourites in the range of Johnnie Walker but all its blends are excellent products, all the age expressions. I have found particularly good samples of the Red Label in recent years.