Diahann is a 20th-century American invention — combining Diane (from Diana, the Roman moon-goddess) + Ann (Hebrew Hannah, "grace"). Oscar nomination for Best Actress for Claudine (1974). Grammy nomination; daytime Emmy.
Subject of her memoir The Legs Are the Last to Go (2008).
Diahann does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Diahann reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.