I have used this product on a few swords but use with caution. I too was shocked the first time that I used it. It leaves a horrible dull grey surface.
Be prepared to do a bit of polishing after treatment. I use very fine carborundum paper, used wet, and then polish with Solvo.
I wouldn't use on a sword with faint etching because the polishing will wear it further. Neither would I use it on any sword with blueing or gilding. On a plain blade it is fine but go carefully with the treatment. I apply the liquid with a brush, leave for half an hour and then wash in water using a brass brush on the rusted areas. Repeat if required.
I don't like seeing rust on any sword because it is damaging the metal underneath. Black patches also harbour red rust underneath so it isn't safe to leave that on the metal either.
I contacted the makers of De-corroder a year ago regarding the effect their product had on metal. Surprisingly the were quite taken aback and told me to stop using it.
Ian