Preheat and make the spice rub: Preheat the Blackstone to low. If possible, allow the ribs to warm on the counter for 30 minutes for even heating. In a small bowl, mix together the brown sugar, paprika, onion powder, garlic salt, sea salt, and ground black pepper. Set aside.
Remove the membrane: Remove the thick white membrane from the underside of the ribs. Loosen it carefully with a knife as needed. Once loosened, pull it away from the rack and discard.
Coat in mustard: On a foil-lined baking sheet, spread the mustard evenly on all sides of the ribs.
Add the dry rub: Generously sprinkle the spice mixture onto each side of the ribs. Rub into the ribs, ensuring the edges are coated.
Wrap in foil: Wrap the ribs in the foil tightly and place them onto the Blackstone with the bone-side down. Close the lid and allow to cook for 30-45 minutes.
Cook to temperature: Carefully flip the foil package over and cook for an additional 15-20 minutes until the internal temperature is between 190°F-200°F.
Sear the ribs: Increase the heat to medium. Remove the foil from the ribs and sear on each side for 5-10 minutes until browned. You may need to press down on the very edges of the ribs to ensure they make contact with the flat top.
Caramelize: Brush each side generously with barbeque sauce. Then, turn and allow each side to be seared with the sauce for 2-3 minutes until it begins to caramelize.
Add the final sauce layer: Next, brush more sauce over each side of the ribs as desired.
Rest and serve: Allow the ribs to rest for 10 minutes. Then, slice and serve warm.