Just continuing my current treatise on Joker here for a second.
Joker killed all his old henchmen with Red and Black toxic roses. Obviously, as we learned, the whole "red & black" motif was all meant for Batman. And it took me a long time to figure out the gag. Obviously, there was the checkerboard pattern which forced Batman to look for a pattern or game. But beyond that? There was something simpler that I'd missed.
Black Roses: Symbolically, Black Roses signify lots of things. Black Magic (Okay, the Devil is involved here). They're also a symbol of the anarchist movement (which is insanely appropriate for Joker) But in the 18th Century, according to the Language of Flowers, Black Roses signify a few things.
In the 18th century, the language of flowers became popular. In this code, black roses to most people signify death, or hatred. But it also means farewell. Black rose means revenge to a foe or wanting to kill someone. However, because a black rose is virtually impossible to procure, it can also mean pure love. A black rose can also mean rebirth, though this is less known.
Now ... this speaks volumes about Joker. He's Batman's arch-nemesis. He's always trying to get "revenge on a foe" and "wanting to kill Batman". And he does hate Batman. And more than that, he IS bidding Batman farewell. But he also loves him. Which is why he also uses ...
Red Roses are a symbol of true love. And we all know part of The Joker's twisted M.O. is that he feels like he's the only person worth Batman's time, and vice-versa. Without Batman there's no Joker, and without Joker there's no Batman. It's partly narcissism, and partly vanity.
Of course, to top it off ... Red/Black is a complicated system the NSA uses to section off where in cryptic, coded messages the sensitive information is stored. So it seems right up Joker's alley.
And to top THAT off ... the French novel The Red and The Black is a psychological drama that highlights the inherent dichotomy between thinking (black) and feeling (red), although ultimately any other comparisons would prove circumstantial.
And of course, there's the Blue Oyster Cult song of the same name ... but that's about Canadian Mounties.