The word "black" is there because many years ago (say 40 years or more) the finishing coating was a hand of a tar based rust preventing paint. As tar is black, the finishing color was also black. Nowadays, as G. Roberts says, there are more modern coatings, but the word black is still there.
Hot dip galvanized means that the just rolled (or extruded) pipe is submerged for a few seconds into a bath of molten zinc.
They're two different finishings, hot dip galvanized being the more expensive, of course.
Giovanni S. Crisi
Sao Paulo - Brazil