document.getElementById("tblBlah").rows[i].columns[j].innerHTML;
Should be:
document.getElementById("tblBlah").rows[i].cells[j].innerHTML;
But I get the distinct impression that the row/cell you need is the one clicked by the user. If so, the simplest way to achieve this would be attaching an event to the cells in your table:
function alertInnerHTML(e)
{
e = e || window.event;//IE
alert(this.innerHTML);
}
var theTbl = document.getElementById('tblBlah');
for(var i=0;i<theTbl.length;i++)
{
for(var j=0;j<theTbl.rows[i].cells.length;j++)
{
theTbl.rows[i].cells[j].onclick = alertInnerHTML;
}
}
That makes all table cells clickable, and alert it's innerHTML. The event object will be passed to the alertInnerHTML function, in which the this object will be a reference to the cell that was clicked. The event object offers you tons of neat tricks on how you want the click event to behave if, say, there's a link in the cell that was clicked, but I suggest checking the MDN and MSDN (for the window.event object)