Change the master page from code behind

Set it in the Pre_Init event:

void Page_PreInit(object sender, EventArgs e)
{
    MasterPageFile = "~/Blank.master";
}

See http://odetocode.com/Articles/450.aspx for some detail and more options.

put the following line in the Page_PreInit method of your codebehind page:

protected void Page_PreInit(object sender, EventArgs e) 
{ 
    this.Page.MasterPageFile = "~/Blank.master";
}