Bring a large pot of water to a boil. Add the purple cauliflower and broccoli florets and blanch for 2-3 minutes until bright in color and slightly tender. Drain and immediately transfer to a bowl of ice water to stop the cooking process. Let cool for about 5 minutes, then drain again and pat dry.
In a large mixing bowl, combine the cooled purple cauliflower, broccoli, grapes, raisins, and red onion.
In a separate small bowl, whisk together the mayonnaise, apple cider vinegar, honey, salt, and black pepper until smooth.
Pour the dressing over the vegetable and fruit mixture. Toss gently until everything is well coated.
If using, sprinkle the sunflower seeds on top and give it a final toss.
Cover the salad and refrigerate for at least 30 minutes before serving to allow the flavors to meld.