Use CSS as mentioned below:
.modalBackground
{
background-color:filter:alpha(opacity=70);
opacity:0.7;
}
<ajaxtoolkit:modalpopupextender
id="ModalPopupExtender"
runat="server"
backgroundcssclass="modalBackground"
cancelcontrolid="CancelButton"
dropshadow="true"
okcontrolid="OkButton"
popupcontrolid="Panel1"
popupdraghandlecontrolid="Panel3"
targetcontrolid="lbn_job_det">
</ajaxtoolkit:modalpopupextender>
Source @
Modal Popup Extender Basics[
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