Hi Frederic,
Indeed you can apply function on a selection. In fact, this is the default behaviour as soon as you have an active selection in the selection bar.
For Blank value, you just have to click on the white part of the quality bar for the column of interest, then select all the rows with empty value. Then chose the function, and by default it will apply only for the empty values. Same when you select a value through search, by double clicking on the cell, or through the visualization panel.
By the way, I suggest you look at the getting started guide, where we show some of those actions ()
