Kivy: how to set checkbox to checked on start up

How can I set the status of the checkbox with the id set to blue to be checked on start up. I use python 3.6 and Kivy 1.9.2.dev0. I thought the lines blue = ObjectProperty(True) in .py and value: root.blue in .kv would do that but apparently I am misunderstanding how ObjectProperty works

import kivy
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty
from kivy.core.window import Window

class SampBoxLayout(BoxLayout):

    # For radio buttons
    blue = ObjectProperty(True)
    red = ObjectProperty(False)

class SimpleApp(App):
    def build(self):
        Window.clearcolor = (1, 1, 1, 1)
        return SampBoxLayout()

sample_app = SimpleApp()
sample_app.run()

The .kv:

#: import CheckBox kivy.uix.checkbox


<SampBoxLayout>
    orientation: "vertical"
    padding: 10
    spacing: 10
    BoxLayout:
        orientation: "horizontal"
        size_hint_x: .55
        Label:
            text: "Favorite Color:"
            color: 0, 0, 0, 1
            size_hint_x: .265
        Label:
            text: "Blue"
            color: 0, 0, 0, 1
            size_hint_x: .15
        CheckBox:
            group: "fav_color"
            id : blue
            value: root.blue
            size_hint_x: .05
        Label:
            text: "Red"
            color: 0, 0, 0, 1
            size_hint_x: .15
        CheckBox:
            group: "fav_color"
            value: root.red
            size_hint_x: .05