Black caught Vegeta off guard with the ki blade, Vegeta was also likely holding back at least initially since even Base Black was holding his own against him and Rose is presented as being at least a somewhat considerable multiplier.
Besides Black is bias towards Goku and since it is his body vs itself it probably adapts easier, even in SSJ2 Goku was able to push Black to new heights.
Goku probably wins though due to generally being a bit of a better fighter.
Trunks is pretty impressed by Goku's power though, and says even Goku wasn't powerful enough. Seems like he holds Goku in a higher regard, and I don't think he has that bia towards Goku like he used to have in the Cell Saga.
I think I'd agree that Black is biased towards Goku though. Fighting him and copying his style probably helps him unlock more of Goku's power. I think that's even the reason he powered up so much in their first fight even though he never went all out.
SSJR's multiplier is a bit of a mess tbh. It is Black's version of SSJB, but he doesn't look like he got thousands of times stronger. And it doesn't look like Blue Vegeta was suppressed to base level or anything either because a) that's stupid and b) Black was already way beyond base level.
I have:
Goku SSJB (Peak Arc Zamasu) > Merged Zamasu Halo > Merged Zamasu >>> Vegeta SSJB (Post Rosat) >= Goku Black SSJR (Enraged) >> Future Trunks SSJRage = Goku Black SSJR (Zenkai Power) >>> Goku SSJB (Enraged) > Goku Black SSJR (FP) >>> Goku Black SSJR (Suppressed) > Goku / Vegeta SSJB (Arc Zamasu).
Goku Black gets some 3-4 power ups fighting the Saiyans. Goku and Vegeta were fighting him pretty well alone in their first encounter, but in the second he powers up and tolls everyone at once.
Goku gets his rage boost, but even then Black didn't take too much damage. Black powers up and finishes Goku easily.
When Trunks transforms, it looks like he takes a minute to get used to his new form, but Yajirobe says they're equals. Black looks like he got another boost however, since he ended up overwhelming Trunks and made him put all his power into a Masenko to allow for Goku and Vegeta to escape.
I'm not sure if Trunks is stronger than before since he relies on strategy to keep up, and after he blasts Black to hell he's pretty worn out. Black probably powers up from that Garlic Gun he took though.
Post Rosat Vegeta is clearly much stronger than Black, but when Black does his final power up Gowasu says he's the strongest warrior, and he saw Goku vs Hit on GodTube. A producer of the show has called Black the strongest warrior after Beerus as well when this episode was released.
SSJR Goku Black (Peak) > KKx10 SSJB Goku (U6) >=< SSJB Vegeta (Post Rosat) >>> SSJR Goku Black (Post Trunks 2) > SSJR Goku Black (Post Trunks 1) > SSJF Trunks = SSJR Goku Black (Post Goku) >> SSJR Goku Black (1st power up) >= SSJB Goku (Enraged) > SSJR Goku Black >= SSJB Goku >= SSJB Vegeta >= SSJ2 Trunks >/>= Future Zamasu.