Take the polish up a bit to 320 grit. Some got to 400.
The coarser the polish (grit) the blacker the 'rust blue' finish generally appears. Just guessing because it doesn't reflect as much light & color back.
Some soln. just naturally give a different shade than others. The type of steel in the barrels effects the finish too.
Most rust coatings will turn over at less than a boil (212F). But at least give 'em a chance and bring the tank to a soft boil.
Make sure you are getting a thorough carding with the steel wool. It can work, but sometimes isn't as effective as a wheel, at least that's what I've found. That can effect the final finish usually leaving it dull.
