"Every brewer who produces black beer uses some form of coloured malt. It is true that of late years caramel has in some districts partially superseded these, but even the most enthusiastic advocate of caramel will hardly claim to produce a satisfactory black beer without, at least, some proportion of coloured malt. And whilst much has been written and said of pale malts, comparatively little attention appears to have been paid to the small yet quite important matter of the character of coloured malt. Ever since black beers have been produced, coloured malts have been made and used. Indeed the name " Patent Malt," which is still often applied to black malt, is derived from the fact that consequent upon the passing of an Act of Parliament* over 60 years ago, a licence or patent was required for its manufacture. The coloured malts employed are respectively black, brown, amber, and crystal.
Brown and amber malts have of late years fallen somewhat into disfavour, black being relied upon for colour, crystal for flavour. There is. however, latterly a tendency to employ an increased proportion of brown and amber malt, and without doubt such malt if really well made gives a characteristic flavour not possessed by either black or crystal. It is, indeed, by a skillful blending of the several types of coloured malt that some of the most successful black beers are produced. It is true that in such grists the total proportion of the coloured malts will often be large and the cost price of the beer as a consequence high, but the result of the adoption of such grists generally fully justifies the expenditure."
"Journal of the Institute of Brewers vol. 13, 1907", pages 486-487.
I think the author is wrong about the origin of the term patent malt. I'm sure it was used long before 1842 and refers to the patent Wheeler obtained for his malt roasting process in 1817.
He's right about a decline in the use of brown and amber malt and an increase in the use of crystal. Though London brewers did stick with brown malt for their Porters and Stouts. They also used it in K Ales sometimes. Amber malt became rarer and rarer in the 20th century and by WW II had just about disappeared from London beers.