I wanted to add that there is another possibility - to use the function na()
.
e.g. =if(a2 = 5,"good",na());
This will fill the cell with #N/A and if you chart the column, the data won't be graphed. I know it isn't "blank" as such, but it's another possibility if you have blank strings in your data and ""
is a valid option.
Also, count(a:a)
will not count cells which have been set to n/a by doing this.