As a District Sales Representative that inspected the broken frames, and approved the Schwinn warranty credit for the dealer, I can assure you the that the black frames were not just used for Warranty replacements. If you purchased a chrome Sting frame, or a chrome Sting bike, we replaced any warranty frames with the same finish. You could not order any Schwinn color on a Sting like done on the Paramount program. The Stings were built in batches and chromed or painted in batches. Every Paramount built was a "one off build" so it was easy for the factory to offer the custom paint feature on them. If you were a Schwinn Factory sponsored BMX team anything was possible, including "Custom Frame Sizing". Brad Hughes and Russ Okawa ran the Schwinn Factory program. Russ originally worked for Bert Straub and ran one of his Canoga Cyclery locations before Brad hired him.
The frame tubing and the forks were Schwinn factory sourced from Tange. The frame tubing is "scary thin" lightweight, and it is butted (thicker on the ends, thin in the middle of the tubes). Many years ago, I had a chrome Sting frame that was cracked at the seat tube and I replaced it under the Schwinn Warranty. I cut the frame tubes in half lengthwise at the joints for a sample to show how it was made. I had painted the inside of the frame tubes to show the butting points in white and yellow. I'm sure that frame still exists somewhere today with a Vintage BMX guy.
John