Call C# to Javascript in ASP.NET

namespace ArduinoTest
{
    public partial class LEDTest : System.Web.UI.Page
    {

        SerialPort ardo;

        protected void Page_Load(object sender, EventArgs e)
        {
            ardo = new SerialPort();
            ardo.PortName = "COM4";
            ardo.BaudRate = 9600;
            //BlueOn();
        }

        [WebMethod]
        public void BlueOn()
        {

            string blue = "1";
            ardo.Open();
            ardo.Write(blue);
            ardo.Close();
        }

<asp:ScriptManager ID="ScriptManager2" runat="server" EnablePageMethods="true"></asp:ScriptManager>
        <script type="text/javascript">


            function StartBlink() {



            }