in other words, when the old style of blueprints start to be referred to mostly as cyanotypes, the latter term would be a retronym.

Which (I try to understand you well) is the opposite of what McMama is asking for? Yet it seems a bit strange that a word that catches up with a new appearence of a concept/word ( blueprint-cyanotype) is called 'retro'.
Would it not fit better a word that keeps its old appearance when having evolved to a new meaning? (excuse my grammar)