Bring a large pot of salted water to a boil and cook the pasta according to package directions until al dente; drain and reserve a little pasta water if needed.
While the pasta cooks, heat a skillet over medium heat and add the blackened chicken; cook 1–2 minutes per side just until heated through, then remove and slice.
In the same skillet over medium heat, melt the butter and add the pressed or minced garlic; cook until fragrant and bubbling, about 30–60 seconds.
Add the heavy cream, softened cream cheese, grated Parmesan, and Italian seasoning to the skillet and whisk until the mixture is smooth and combined.
Increase heat to medium-high and bring the sauce to a rapid simmer; let it boil gently 1–2 minutes while stirring occasionally, then remove from heat — the sauce will thicken as it cools.
Taste and season the sauce with salt and black pepper as needed, then toss the drained pasta with the Alfredo sauce (use reserved pasta water to loosen if necessary) and top with the sliced blackened chicken before serving.