You don't say what should trigger the messages to appear, so I'll assume each will display for 2 seconds. Basically you want to invoke blockUI again onUnblock of the previous message.

$.blockUI({
    message: '<h1>hello world!</h1>',
    timeout: 2000,
    onUnblock: function () {
        $.blockUI({
            message: '<h1>hello galaxy!</h1>',
            timeout: 2000,
            onUnblock: function () {
                $.blockUI({
                    message: '<h1>hello universe!</h1>',
                    timeout: 2000
                });
            }
        });
    }
});