First I'd like to preface something. Notice in both the Dragon Ball Super manga and anime they constantly mention Trunks' timeline as being a "Parallel Reality" or a "Parallel World" so to speak. This will be important, cause I think this is one of the key elements to distinguish what's going on with someone like Black and what's going on with someone like Trunks or even Cell.
Judging by the Time Rings, we know there are 5 of them in total. In the Dragonball Super manga, it was mentioned that the very first one came from someone in Universe 12 a long time ago. The other three Time Rings were created by Trunks and Cell clearly. The silver Time Ring however appears to be the "main" Time Ring of the 5, though it appears that it's able to travel through every timeline as Black mentioned in the manga he's finally found a timeline where Beerus is dead.
Still, this doesn't quite explain the situation with Black, but I'd like to explain another concept here called Schrodinger's cat. Most of you probably know of it already, but if you don't, here's an excerpt from wikipedia about it:
Basically it says because something CAN happen, it WILL happen, no matter how many different scenarios are involved. Also linked to the many worlds theory or perhaps the multiverse theory.Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that may be simultaneously both alive and dead, a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur. The thought experiment is also often featured in theoretical discussions of the interpretations of quantum mechanics.
So here's my theory: What happened with Black is not a separate "Parallel Reality" like the five we got right now, but rather it's one of many possible timelines that happens to connect itself with another possible timeline by Black's travels to Future Trunks' "Parallel Reality." While Trunks does time travel, he doesn't actually create a new timeline more-so than he creates a new reality itself.
This is why we got things like all the movies that don't quite fit, Dragonball GT, Dragonball Xenoverse, Dragonball Heroes, etc. They all operate under the same base concept here. Essentially if you think about it like this, there really was no stopping Black from ever entering into Trunks' timeline cause even if you manage to stop him like Beerus did, because there was a possibility that Zamasu actually initiate his plan, he DID initiate his plan in "another timeline" or "another world."

