JF Ptak Science Books Quick Post
Advertisers for Henry M. Stanley's Through the Dark Continent could not paint Africa any darker than they did in this ad which appeared in the journal Nature on May 3, 1878. Stanley plowed his way through Africa from 1874 to 1877, and was quickly into print on his return, which may have been more armed Imperial Pushing than it was a geographical expedition.
Stanley published another book, In Darkest Africa. twelve years later in 1890, no doubt alluding to the unimaginably dark places of a dark place.
And just for the sport of it I had a quick tour around the massive 75,000,000-item database of library holdings in WorldCat and found that there may be only one other book earlier than Stanley's published with dark/darkest Africa/Continent in the title (Christian Liberia, the Hope of the Dark Continent, with Special Reference to the Work and Mission of Edward S. Morris, 1838). After Stanley's book was published there are easily dozens of other books using that phrase published between 1878-1900. So there's that. In my long career of looking at maps I cannot recall the continent appearing on world maps as simply an entirely blacked out/filled-in geographical region--certainly in earlier maps of Africa (or at least before 1900) the unknown spots were simply left blank; the older the map the more likely it was to be that the blank spots were filled in with unusual representation of flora and fauna. But never, it seems to me, was it just entirely black.
The full add looks like this: