As I understand, Melonite is highly similar to Glock's Tenifer.
It's not just a coating, it's a metal treatment. The steel on the surface and down into the steel a few millimeters is rendered much more resistant to corrosion and much harder on the Rockwell hardness measurement. Then a standard black parkerizing is added to that. So even if, say, an older Glock looks worn, you're just seeing wear to the parkerizing, not the actual Tenifer finish.
I think Melonite is just another brand of Tenifer and is basically identical.
I may be wrong about all of this, but that's what I understand of it.