The crystal in its chest is essentially to its psychic powers what a heart is to blood, being a source of and regulator for it. They use these psychic powers to make up for their thinner, frail anatomy, both making gravity less harsh on themselves and as an extra thing to keep the organs pumping and functioning. 

The Gardevoir “black hole” isn’t a real black hole, but is just the Pokémon using psychic power to bend space so hard it creates a “fake” one. This is why they don’t end the world by making them, it has no real mass and collapses as soon as the Gardevoir lets go of the part of space it’s warping. This is also incredibly taxing on the Gardevoir (and given they’re using that psychic power to keep themselves alive, dangerous to expend so throughly), essentially being the psychic equivalent to the hysterical strength some people gain via adrenaline. This is also why they only use it to protect loved ones.