the "blocking object" is not work

hi guys, I tried to add a button in my scene. in the canvas of the button, I set "blocking object" to "all" in the graphic raycaster. In the scene,there is a gameobject with a script,simply code:

if input.getmousebuttondown(0) doSomething();

but when I click on the button

I found the click is not blocking,the "dosSomething" is still called

is that a new problem for unity5？