Display alert

I am trying to display alert message depending on recCnt value as below:

ScriptManager.RegisterClientScriptBlock(Page, typeof(Page), "alertMsg", "SaveAlert('" + recCnt + "')", true);

JS:

function SaveAlert(rst) {
if (rst == '-1')
alert('Selected Item already there in your Favorite list');
else if (rst == '-2')
alert('Selected Item already there in your Blocked list');
else
alert('Record Saved Successfully');
}

Though it's displaying message fine but sometimes it displays message and sometimes not. Moreover, it completely blackens background. What can be the best way to display alert?