Dark Matter for a Black Heart
The little piece of pure evil featured in this episode made its first appearance in House of Secrets #61 in 1963. The Heart of Darkness has torn its way through the DC Universe for years, and its origins have morphed timed and time again, but at some point it fell into the hands of Bruce Gordon (the combination of Bruce Wayne and Commissioner Gordon), who would become the villain Eclipso.
“I know, I know, I can’t smoke here.”
This is a nudge-nudge, wink-wink jab at NBC, who told Constantine that they couldn’t film John smoking. To be fair John didn’t inhale. No, really. He let the cigarette dangle from his lips.
Did we see a Flash/Constantine crossover?
Not exactly. The ill-tempered patient Morris was reading a science magazine in the hospital, and the title of magazine was the same—but the issue different—as the one Barry Allen used to shield his head from rain in The Flash pilot.
Coming Around Full Circle
The cover story in the science magazine Morris is reading just happens to be written by Bruce Gordon (aka the future Eclipso).