What is the Go-To black finish?

[VictorLouis]

Years back, I'd sent off a m28 for boron-carbide Diamond-Like-Coating.
Is that still the one? If so, who's the compentent vendor/s?

 

[GerSan69]

I suggest H&M Blacknitride+™ | The New Standard in Steel Hardening Their finish is the same as the finish on S&W semi-autos. And they have a factory right here in Glendale AZ.