e.TableCellIdentity.SummaryColumn is only set if th eelement is a captionrow or summaryrow. So, if you want this information on recordrows, then you would have to track it yourself.
You could add a private ArrayList member, and add the column names used in your summaries to the ArrayList as you add the columns to teh summaryrow. Then in QueryCellStyleInfo, you could use code like this where colsInSummary is the arraylist:
//blanks out the summary column
if ((e.TableCellIdentity.TableCellType == GridTableCellType.RecordFieldCell
|| e.TableCellIdentity.TableCellType == GridTableCellType.AlternateRecordFieldCell)
&& this.colsInSummary.IndexOf(e.TableCellIdentity.SummaryColumn.Column.Name) > -1)
{
e.Style.CellValue = "";
}