Black rust

[Greybeard]

I encounter this problem for nearly 40 years -- first with my early Colt automatic pistols, now with some of my keris blades: spots of black rust!

While red or brown rust usually comes off easily, black rust is very hard and very stubborn and seems almost impossible to remove. Is there any method to get rid of those ugly black rust spots? Any suggestion will be appreciated.

Regards,

Heinz