Build the Wet Rub: In a large glass bowl, combine the olive oil, fresh citrus juices, dark chili powder, cumin, minced garlic, salt, and black pepper. Whisk vigorously until the heavy spices form a thick, dark marinade.
Coat the Meat: Toss the completely raw chicken thighs directly into the dark wet rub. Massage the meat until every single piece is totally covered in the heavily spiced, sticky layer.
Sear the Chicken: Heat a large cast-iron skillet over high heat until fiercely smoking. Drop the coated chicken thighs into the totally dry pan and cook completely undisturbed for exactly 5 to 6 minutes per side until deeply charred.
Chop the Meat: Pull the violently hot chicken out of the pan and let it firmly rest on a cutting board for 3 minutes. Aggressively chop the dark meat into tiny, perfectly even street-style cubes.
Blister the Tortillas: Immediately drop the completely cold corn tortillas directly into the incredibly hot chicken fats left inside the dirty skillet. Blister them for exactly 20 seconds per side until remarkably pliable and deeply stained.
Assemble the Tacos: Stack two fiercely hot, blistered corn tortillas together. Heavily load the charred chicken cubes inside, then aggressively top with the sharp raw white onions and highly fragrant cilantro. Serve immediately!