I was born in the 70s, raised in Oregon, and we called them chalkboards. "Blackboard" sounded old-timey to me, like something you'd hear on Little House on the Prairie. A Google search result points out that green became a common color for chalkboards, and in retrospect I think most of mine in school were green.
More googling says that the original blackboards were slate. The green ones were porcelain.
Our green ones varied in quality and material. Most of them seemed to have some cheap coating that was sometimes torn by chalk; after being torn, the edges of green coating seemed stretched like thin plastic. Some of the green ones, the teachers actually liked. Maybe those were porcelain?
Also had blackboards at some of my schools. They seem to last forever without much care.
More googling says that the original blackboards were slate. The green ones were porcelain.