Re Black a Firearm's color (to black,) what is a good product to use for touch up?

[KnightExtreme]

Looking for a good easy to use product to reblacken a firearm? Any that good / durable and easy / simple to apply with no baking required?
Any suggestions?
Disregard the Circles (in the pic) as those are the 2 pins required to remove this Trigger Assembly to get into and clean up the rust and then refinish with some black coating.
Was looking at maybe " Birchwood Casey Long-Lasting Fast-Drying Super Black Touch -Up Pen for Deep Scratches and Worn Areas, Flat Bleck".

https://www.amazon.com/gp/product/B00JSAE0ZY/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1​

Thanks,
KE

 

[Taudisio]

How much do you need to cover and what does it look like? Covered in rust? Just bare metal? Is it nitrided, blued, parkerized, or something else? I have often used the little sharpie for small touch-ups.
Can you post photos of what needs fixed?