The Blue Diamond is a golden age hero, but only barely. Elton T. Morrow made only a couple of appearances as the Blue Diamond in the 40s but it was enough to get dusted off for an appearance every thirty years or so. Technically he’s still active, but he’s not due another comic run until the 2030s.
References: [wikipedia.org: Blue Diamond]
[marvel.com: Blue Diamond]
[mavel.fandom.com: Blue Diamond]
[crystal]
Metahuman Database 