I didn't feel Dayna was ever defined by her ethnicity in Series C, but there were a couple of Series D episodes where it was called attention to. In "Power," one of the Seska referred to Dayna as "the black woman," even though she was the only non-Seska woman in the scene so they didn't need to specify. And in "Traitor," when they were discussing going undercover on Helotrix, they had that awkward dialogue exchange where Dayna wondered if she could pass as a native and Avon said, "No problem. When Helotrix was first settled, the old Stock Equalization Act was still in force. Every Earth race had to be represented."
I read that after B7, Simon became the first black member of the Royal Shakespeare Company, and stayed with them for decades.