Blocksters are small social monsters, they need company of their own kind. Your goal is to make them happy and to achieve as many score as you can!
The problem is, they are appearing all the time. You need to be quick and have to make smart moves, otherwise the field will quickly fill up with blocksters and the game will be over. They need place for moving, so don’t let them overrun you. Collect 4 or more blocksters of the same type - that will make them happy and they will disappear.
It helped me to group my sprites to sprite sheets and optimise the game