Peel, clean and devein the shrimp. Rinse under cold water. Butterfly open and transfer to a glass or ceramic bowl.
In the blender combine 2 roughly chopped serrano peppers, garlic, small handful of cilantro, and the juice of 3 limes. Season, to taste with salt and pepper. Blend on high until smooth.
Slice the cucumbers and the red onion. Set them aside. To the bowl of raw shrimp, season lightly with salt and pepper. Pour in the blended aguachile. Stir well to combine. Chill for 10 minutes.
When ready, add the onion, cucumbers, 1 tbsp. of finely chopped cilantro, and salsa negra, if using. Stir well to combine. Chill for another 10 minutes. Serve with sliced avocado and tostadas or chips.