This is sort of a pedantic question - the kingdoms of the Korean peninsula took their cues from China and Balhae was no exception.

Was Balhae "Korean"? What does it mean to be Korean?

Balhae was made up of the remnants of the Goguryeo kingdom. The Goguryeo were defeated by the unified Silla/Tang army with their kingdom from Pyongyang to Wonsan south going to Silla and everything north of that becoming Balhae. This land includes all of North Korea from the Pyongyang-Wonsan line north, as well as most of Liaoning and Jilin provinces, and a small portion of Heilongjiang province. By modern standards and political boundaries, the bulk of the land is Chinese. But at the time, this region was populated by the Malgal people, a semi-nomadic people that ranged from Siberia, to Manchuria, and into northern Korea. These people were decidedly neither Chinese nor Korean, but they had lived under the Goguryeo rule and then they fell under Balhae rule.

Balhae was founded by a former Goguryeo general, Tae Cho-yong, who became King Ko. Ko and his kingdom faced aggression from both China and the unified Silla and they responded with aggression. The Silla built a defensive wall along their norther border and King Mu (719-737) sent raids against Tang China, attacking the Shandong peninsula by sea.

Balhae and Tang China reached peace and Balhae began to assimilate Chinese institutions and culture. Under King Son (818-830), Balhae was at it's peak and it reached from the Amur River into South Hamgyong, and from the Russian Maritime territory to the Lioaning Peninsula. Balhae, as well as Silla, had strong economic and cultural ties to China. Both kingdoms were part of the Tang tributary system and exchanged goods with China under this relationship. Also, Silla and Balhae monks and students went to China to study Buddhism and Confucianism, bringing these new ideas back to their respective homelands.

The best way to characterize the relationships is to say that Goguryeo/Balhae was a tribe that was culturally related to the other tribes of the Korean peninsula, but that all the Korean tribes took their cultural cues from China.