Blue prints are given out based on the age of the building you motivate/polish/plunder. If it is an Early Middle Age building, you will randomly receive a blue prints for one of the two Early Middle Ages Great Buildings. The chance to receive a blue print is random and individual for each action, meaning 200 actions does not ensure you a blue print. Think of it as a coin toss, each time you do an action you have a chance to receive a blue print (but obviously the chance is very small, blue prints are rare). That chance stays the same for each action you take, but obviously with more instances of the same chance you have a bigger probability to receive a blue print.
There are guilds who have tried collecting data and presented which building are best to do actions toward, or at what time of the day, in order to receive blue prints for specific blue prints. The idea is wrong, because that is not how the drop system works, though since the data was based on their own drops, the data is not wrong for them. But players should not lead themselves by it. I myself have tried recreating such data and it proved me it is wrong.
