Hermes calls the outline screen the finesse screen. Is it printed with a different ink ? Maybe It is used to isolate regions of different colors - that might bleed unattractively if they touched. So, the black line allowed the colors to bleed onto black. The finesse ink does not wash out in the process. Obviously, now sometimes the Finesse screen is not black , gray is popular
But a finesse screen isnt what it used to be. The signature and copyright used to be on the finesse screen - that is no longer the case, and they (definitely) get special inks & screens and few artists really use a finesse screen, at least not to great advantage. Faivre was a genius at milking the finesse screen - an oldie Art des Steppes ca 1991. The whole design is on the finesse screen - every color is separated.
Compare to the recent Honore - PdS. The outlines dont define the design as much - a lot of shading. So, 30 years later the finesse screen does a lot less. And for amusement I tossed in a photo of the Red PdS in CS.
And PdS is indeed screened, I cannot say that for certain of many of the modern ones