I believe that Beretta referred to that flat black as a black chrome finish.
It was used on the barrel of a 303 auto I had and I seem to remember an older 686 Onyx that also had it.

It was very effective at avoiding reflected glare and held up well to use/abuse/cleaning on the guns that I had with it.

There are similar finishes being done to aftermarket handguns by the custom houses.

Courtesy of a WOMBAT
* (Waste Of Money Brains And Talent)

YMMV