Hoping the upcoming episodes give us some solid info about this mystery, but right now I believe that the Zamasu from the present traveled to Trunks' future and created Black there. Of course, the
how is the where the major question lies. Given that we know Black has similar ki to Zamasu, and his comments about fighting Gokuu in "this body", some thoughts:
1. Zamasu is on track to be come a Kaioushin, which are gods of creation opposite the gods of destruction. This is usually said to imply that they create new planets, but do we know if they can also create living beings? If it's never been stated that they can't, Zamasu could have simply used his power to create Black, imbuing him with all his thoughts/feelings about mortals, molded in the image of the human that treated him rudely.
2. Another thought is that there are two Zamasus involved: the one from our timeline, and the one from the future. The present one travels to the future, tells his future self about his conclusions about humanity being worthless, and they work together to create Black, with one of them becoming the base from which he's formed. Though since Black's ki is not exactly the same as Zamasu's, it means either his ki is somehow corrupted in the process of becoming Black, or perhaps it's been combined with another, which leads me to...
3. Whether two Zamasus are involved or just one, it's possible that Gokuu's body was taken from the afterlife in the future to help create Black. If there's one Zamasu involved, he could've captured him and infused his own ki into him, taking control of his body and mind (though that would feel like it's seeing up the possibility of Gokuu breaking free of his control and regaining his conscience, which I don't think I want to see). If there're two Zamasus, one of them could combine with Gokuu (in some process other than fusion, since it doesn't look like the result of that), resulting in Black. I feel that Gokuu's body/mind being involved is highly probable, since Black recognized Vegeta, whom Zamasu never met.
Whether my theories are on track or completely wrong, I just hope the eventual explanation is both shocking and makes perfect sense