Yes you are right.. Using ContentPageID_ControlID not good,
But we have an option try like below it will help you...
On the Submit Button Client Click Event call a user defined function and on that function try the below code...
JavaScript :
var elems = document.getElementsByTagName("*");
for (var i = 0; i < elems.length; i++) {
if (elems[i].id.indexOf("txtBloodGroup") != -1)
{
if (elems[i].value == "") {
alert("Please Enter Blood Group Name");
elems[i].focus();
return false;
}
}
}
Also, Read this
Finding Controls in a Master Page with jQuery Article, Here they discuss about this issue and solution....