The connections between Goku Black and Reverse-Flash are that both are narcissistic, twisted versions of famous heroes (Goku and Flash, respectively) who hail from the future, are named after their respective heroes along with an additional word to signify their villainy, and have an obsessive desire to ruin their lives. Both were respected members of their universes (a Supreme Kai in training and a scientist, respectively) before turning to evil out of a fear of something related to the heroes they oppose (Zamasu feared and envied the potential of mortals like Goku while Thawne learned of his destiny to become the Flash's nemesis). Both possess the same powers as their enemies which they gained by taking something from them (Goku Black took Goku's body and Reverse-Flash took the Silver Age Flash costume) and can use a form of negative/corrupted energy (Super Saiyan Rosé and the Negative Speed Force). Both have killed the mother of great heroes (Trunks' mother Bulma and Barry's mother Nora Allen, respectively) and the wife of the heroes they oppose (Chi-Chi and Iris West, respectively). Both also possess time travel abilities, which they have used to wreak havoc on their respective timelines.
Both also temporarily became semi-immortal. Goku Black fused with Zamasu to become immortal, and Reverse-Flash became ungrounded from time in Flashpoint, meaning that every time he died, he still existed.