You can have multiple statements within a for each row() clause. I.e. For Each Row( If( ... ); If( ... ), ... ).
Or perhaps better, loop over a list of columns (inside the For Each Row()-loop or stand-alone) as in the example below
dt = Current Data Table();
cols = dt << get column names;// or any list of character columns
For Each Row( For( i = 1, i <= N Items( cols ), i++, If( cols[i][] == "", cols[i][] = "Blank" ) ) );
// Alternatively...
For( i = 1, i <= N Items( cols ), i++, cols[i][dt << get rows where( cols[i][] == "" )] = "Blank");