In a blender, combine sour cream, fresh cilantro, lime juice, and garlic (if using) to create a creamy cilantro lime sauce. Blend on high until smooth, about 30 seconds. Set aside to chill.

In a mixing bowl, mash the black beans with a fork until desired consistency. Stir in taco seasoning and salsa until well combined.

In a large skillet, heat enough oil to cover the bottom, about ¼ inch deep, over medium heat until shimmering, about 5 minutes. Test with a small spoonful of black bean mixture; it should bubble immediately.

Take a tortilla and spoon a generous amount of the black bean filling onto one half, fold to create a half-moon shape. Secure edges if desired.

Carefully place tacos in hot oil, fry each for about 3-4 minutes until golden brown and crispy, then flip and fry the other side for another 3-4 minutes.

Use a slotted spatula to remove tacos from oil and place on paper towels to drain excess oil. Serve immediately with cilantro lime sauce on the side.