Me too B-Dubs, as much as I love Blue Exorcist, it does tend to put me to sleep and Black Lagoon is the perfect change of pace pick-me-up. I seriously can't figure out why people don't like it.
Black Lagoon is totally a product of the 90's and Tarantino movies, it goes against the grain of modern anime to an insane degree. Modern anime is often closer to Blue Exorcist, flashy fights with some moe characters sprinkled in for good measure. Even Attack on Titan follows this basic formula, with a spoonful of giant zombies for flavor.
You'd think this would mean it would be viewed as a breath of fresh air and would be popular, but the current fanbase has been brought up to expect certain things from their anime and Black Lagoon has none of those things. Instead of a strong male MC that they can project themselves onto, they get Rock. Sure he's totally badass, but he's also an adult who has been disillusioned with the world. Often anime is wish fulfillment, an idealized world with characters who can achieve anything if they work hard enough. Black Lagoon is none of those things. Rock's a victim of a fucked up world, he has agency in his daily life but he can really do nothing to change the overall course of it. He's stuck where he is, in a garbage bin, tossed aside by the rest of the world and labelled as useless.
Then there's Revvy, who is totally awesome as shit and easily the toughest of the main characters. While we might appreciate this, the average viewer won't for exactly the same reason almost all of the player characters in video games are male. They're guys and they can't--or rather won't--relate to her at all. They're used to girls in shows being cute, useless and meek. The sort of girls they could see themselves approaching. If you tried to hit on Revvy she'd shoot your dick off.
So while being on late at night is part of the reason, perhaps a larger portion is the average viewer dislikes it for all the reasons we enjoy it. Which both makes no sense and perfect sense.
It wouldn't shock me at all if you told me that Black Lagoon's viewership skews older than the rest of the block.