Dragon Ball Super has spent much of its time expanding both the cosmos and the mythology of the Dragon Ball universe. As if 11 other universes each possessing their own Gods of Destruction, Angels and Supreme Kai weren't enough, the series decided to bring time travel back into the mix for its most complex villain origin story yet. Goku Black's appearance in Future Trunks's timeline is complicated, to say the least. The body-swapping, time-traveling, multiverse-hopping villain has likely made more than a few viewers pause to grab a pencil and paper.
Dubbed "Goku Black" by Future Bulma because he looks like Goku, wears black and is evil, the true identity of this villain is eventually revealed to be Zamasu, the apprentice to the Supreme Kai of Universe 10. But where did he come from and why does he look like Goku? In order to comprehend the series of events that led to Future Trunks's timeline being destroyed again, it's important to understand Zamasu himself.
A Supreme Kai's responsibility to their assigned universe is to kickstart life, then simply observe its progress. Zamasu hails from Universe 10, where he studied to become the new Supreme Kai. However, Zamasu grows to despise the mortals he is tasked with watching over, seeing them as violent, selfish, ugly stains on the beauty of life. This hatred even warps into a hatred for the other gods, who allow the mortals to run rampant because they refuse to interfere. He eventually develops a plan, "Project Zero Mortals," in which he will eradicate all mortals and gods from the entire multiverse.
Despite having an admittedly compelling origin story for a villain, Zamasu's journey from a disgruntled Kai into an evil clone of Goku required a tremendous amount of hoop-jumping. In the main timeline, thanks to knowledge gained from Future Trunks's arrival, Beerus travels to Universe 10 and destroys Zamasu, thus sparing the series' main timeline from his plans. However, in an alternate future where Beerus does not Destroy Zamasu, the evil Kai apprentice kills his master and steals his Time Ring before traveling to Universe Six, home of the Super Dragon Balls.
In this new, alternate future, Zamasu uses the all-powerful Super Shenron to wish to swap bodies with that timeline's Goku. As a Supreme Kai, he is severely limited in terms of combat strength, explaining why he wanted Goku's Saiyan abilities, so that he may carry out his plan himself. Once Zamasu obtains Goku's body and power, he promptly slaughters Gohan, Goten and Chi-Chi before using the stolen Time Ring to travel to Future Trunks's timeline. There, he joins the Zamasu of that time to enact Project Zero Mortals. It is here that he becomes known as Goku Black, as Future Trunks and Bulma are unaware of his real identity.
All that said, it doesn't quite explain why Goku Black turns pink. Zamasu's wish transferred his being into Goku's body, meaning he is now technically a Kai and a Saiyan, with access to all of Goku's Saiyan power. When mortals like Goku and Vegeta use god power to become Super Saiyan, their hair turns blue. However, Zamasu is a god, so when he uses god power to become a Super Saiyan, his hair turns pink, or Rosé, as he calls it. Put simply, Super Saiyan Blue and Super Saiyan Rosé are the same forms, the difference in color just reflects whether the user is mortal or a god.
Dragon Ball has always been a fan of evil Gokus, like the non-canon Turles or when Captain Ginyu also stole Goku's body. Super showed impressive determination to making another evil Goku, folding in the expanded series mythology as well as the return of Future Trunks for good measure. Unfortunately, most of Goku Black's time in the series was spent explaining how he came to be in the first place. Zamasu's plan probably would have worked if he didn't have to spend so much time explaining it to everyone.