Since the merger of Zamasu has appeared there is no greater debate than that of his appearance. First, when the fused villain appeared in episode 64 it was thought that his appearance was more similar to Zamasu than to Black. This has a logical explanation which is that both (Black and Zamasu) are versions of Zamasu of a different time, past or future. That is why we see the fusion with the hair color of Zamasu and his face, are very few features of Black that has this character.

There is another more official explanation below:

The 'pothala arbres' serve to fuse two beings that can be totally different, around body, power, aspect and even race.

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As it was said in the series these individuals can never be separated, nevertheless a desire for the spheres of the dragon can end with the link, like Kibito Kai, as well as the trip to a different dimension as it happened with Vegito.

These arcillos also have a particularity as sacred tools and if any of the two characters that merge are sacred, such as a Kaio Shin, the resulting fusion will contain 80% of the appearance, personality and so on of the sacred person. This happened with the elderly Kaio Shin of 15 generations when he merged with a witch involuntarily.

It is clear that the sacred person in this case was an old man. The same case occurred with Kibito and Shin where Shin was the most sacred being to be Kaio Shin while Kibito was only an assistant.

Let’s analyze

Now with Zamasu's merger, we see that the one that predominates in a sacred way is Zamasu and not Black, since although both are Kaio Shins, Zamasu has the appearance of a Kaio Shin and also did not get his place killing the Shin Kaio in turn. As was the case of Black, a character that could be judged as not sacred.

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That is to say, Black, besides having the body of Goku, got to be Kaio Shin of a dark form, eliminating from its teacher two opportunities, at least Zamasu was it by the death of its teacher made by Black. If there is one who holds more "sacred" signs or sacred features it's Zamasu and not Black. For this reason the merger has an aspect of 80% of Zamasu.