How come researches flash blue sometimes

If you click on something you want to built, but you haven't met the research prerequisites yet, you'll get a message stating that you can't build that because the necessary research hasn't been done. When you do into the Research screen, the item(s) flashing in blue denote what you need to research in order to build whatever it was you clicked. Makes it easier to tell what you need to research in order to be allowed to build whatever it is you're after.