Blackheart is one of the half-dozen Celestial deities, and he is the Celestial of Chaos, Mischief, and Trickery. He just does whatever pleases himself, whether that be slaughtering an entire village, curing a worldwide fatal sickness, or sitting back and watching two birds fight over food. So, it really doesn't matter if it's trivial, evil, graceful, insane, moronic, or whatever else it could be. Obviously, with certain things, the other Celestials keep his chaotic nature in check, so even if he felt like destroying the mortal plane of reality, he couldn't without having a conflict with them. He's typically seen as a neutral deity because of how he could switch from one side of the moral scale to the other in the snap of a finger.
Thankfully, the only really horrible thing the Celestials have allowed him to do is curse a village of horrible people with a slow-acting, fatal illness. There is only one surviving member of that village, and they now worship the entire pantheon of Celestials. At the time, the survivor was a teenage woman that was brought up by thieving parents who also killed a priest of Celestia in another village months prior to the curse, but praying to and begging the Celestials for forgiveness finally got her cured of the illness no more than a year later. Every so often, while she's in her own temple she built for the pantheon, Blackheart comes out of nowhere to scare her, remind her of her past life, cackle, then leaves again, but she's smart not to get mad at him directly, or she wouldn't ever leave that temple alive.
Blackheart is typically seen as a white male, with long, messy, black hair, burning, red eyes, and a sinister smile from ear to ear. He's seen wearing armor made from nature itself, but enchanted by himself and Oracle so that it is almost indestructible by mortal means. He usually holds a set of sharp, double-edged, obsidian blades, but in other depictions, he wields a molten blade with a similar looking shield, with flickers of lighting around them both. In the background is typically a raging lightning storm over an erupting volcano, with a huge wave rising high enough to sweep into the storm clouds.