Fill a large pot with lightly salted water and bring to a rolling boil. Stir in rotini and return to a boil. Cook rotini uncovered, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain well in a colander set in the sink.

Combine blue cheese dressing, mayonnaise, Buffalo sauce, salt, and pepper together in a large bowl. Stir in chicken, onion, bell peppers, and rotini; toss to coat. Cover and refrigerate at least 1 hour before serving.