Combine the olive oil, garlic and Cajun seasoning in a medium mixing bowl. Stir to coat the shrimp. Set aside.
For the green chili cheese sauce:
In a large saucepan, melt butter over medium-high heat.
Gradually whisk in flour. Cook for 2-3 minutes, stirring constantly. Do not allow the flour to brown.
Slowly stir in milk, continuing to whisk until mixture is smooth and does not contain any lumps.
Add the green chilis and bring the sauce to a simmer. Continue to cook for about 5 minutes or until the sauce has thickened, stirring constantly. Remove from heat.
Add the cheddar, salt, and cayenne pepper. Stir until sauce is smooth.
For the nachos:
Preheat the oven to 400 degrees F.
Arrange the tortilla chips in a thin layer in a 9 x 10 casserole dish or on a large baking sheet covered with foil.
Drizzle the chili cheese sauce over the chips and top with half of the Monterey Jack cheese.
Spoon the black beans over the shredded cheese.
Spread the shrimp over the top of the black beans. Sprinkle with the remaining half of the Monterrey Jack Cheese.
Place the nachos in the oven and bake until cheese is melted, about 8-10 minutes.
Top with diced tomatoes, green onions, pickled jalapenos and cilantro.