Drain (and rinse, if you like) the black beans and dump into a bowl. Add the chili powder, cumin, salt, onion, cilantro and lime juice and mash with a potato masher or fork until chunky.
Set a heavy skillet over medium-high heat. Add a drizzle of oil and slide in a corn tortilla - the warmth should make it pliable. Spoon some of the black beans over one half, add crumbled feta and fold the other side over with a spatula, making a pocket. Cook, flipping once or twice (do it across the spine, to keep from spilling), until golden and crisp on each side. Repeat with the remaining filling and serve right away.
Makes 8-10 tacos.