test for null entry always being triggered when tabbing over field or using mouse

Hi I have the following code in my application for a web form that updates an existing record in my SQL database.
protected void Page_Init(object sender, EventArgs e)         {             var onBlurScript = Page.ClientScript.GetPostBackEventReference(TextBoxFirstName, "OnBlur");             TextBoxFirstName.Attributes.Add("onblur", onBlurScript);             onBlurScript = Page.ClientScript.GetPostBackEventReference(TextBoxSurname, "OnBlur");             TextBoxSurname.Attributes.Add("onblur", onBlurScript);             onBlurScript = Page.ClientScript.GetPostBackEventReference(TextBoxBusinessName, "OnBlur");  & ...