There are two different defaults, and you’re only using one of them.
The default value that you set in the call to $wp_customize->add_setting is the default for the customizer. It does not apply outside of the customizer. The default here is what will be used if no setting exists in the database.
The second default is in the get_theme_mod call, and here you’re using a blank value as the default. You need to use the proper default value as the second argument to get_theme_mod.