Black Death
Many years ago, a teacher of mine told me that the rose was a symbol of (among many other things) the Plague. Supposedly when people came down with the Plague in centuries past, and were thus marked for death, they developed blotches on their skin that reminded some people of roses.
If the skeletal horseman of the RWS Death card represents one of the Four Horsemen of the Apocalypse, "Plague" or "Pestilence" perhaps, and the other figures indicate the very democratic nature of that form of death, afflicting adult, child, king, bishop, etc. alike, then the Black Rider image here is probably inspired by the one from the Christian scriptures, in the Book of Revelations, chapter 6. This would be in keeping with A. E. Waite's background and beliefs.
Another name for the Plague was "The Black Death".
The use of this particular rose could of course be a time reference, to the last great outbreak of the Plague in Britain, in Yorkist times.