The Suspicious alien blueprints is a Pre-Hardmode Miscellaneous item dropped by the Meteor Head with a 1/20 (5%) chance. When used, it grants the ability to craft several items, with the text "Learned how to craft some meteor recipes" appearing in chat.

The Meteor Heads don't drop the blueprints anymore. You need to find a Meteor Tower (says the dev) upon finding a meteor for the first time it lands. The blueprints should be in that tower. I've killed almost 2000 Meteor Heads in my playthrough and not a single one dropped the blueprints as of lately.