Web Forms :: JavaScript Alert Message From Server Side Makes Page Blank?

i have a button,i want when a button is clicked a alert message should be display, i wrote the code like this

protected void btn1_Click(object sender, EventArgs e)
{
Response.Write("<script>alert('Your account has been created')</script>");
}

It work well, but when it pop-up background color become gray,i want when it pop-ups,the background should remain same .

like when we click on button which is associated with confirmbuttonextender..the background become fade but it doesn't go blank..