Hello Flavio, A good question. I have old 19th cent pictures of weapons shining in the sun, so some were obviously shown off clean and bright but that is not how collectors want them, strangely if it were a cavalry sabre parade clean is disired. The black from cleaning a white weapon is I think, oxidization and a small amount of etching action by rust. I personelly have not seen blackening by application only from the use of the natural iron scale produced in the forging of the metal this like decorative blueing on military fancy swords offers protection from rust. You will see on your pieces that where the iron scale is left on or used to decorate the piece there will be very little if any rust. The iron scale may be made to shine with the application of some substance but that is not what makes it black. I hope I have not taught your grandmother how to suck eggs
You might not understand that
Tim