I don't think Blockly is what you are looking for.
Blockly is a JavaScript library used to build block-based editors for specific situations. For example, E-Z Robot used Blockly to write an
editor that can be used to control a robot.
Scratch is an editor built with Blockly that can be used to create games and animations. As far as I know, nobody has used Blockly to create an editor that can be used to create a license plate reader.
I think what you need to do is decide what camera you want to use, learn what language can be used to control that camera, and then write a license plate reader in that language. You will need to use a text editor or
IDE to write your code.