When I value-copy ranges where the If-function was used with a return value of "" (empty string), the cells are not blank, hence ISBLANK() = FALSE.
Could you add a feature to remove empty strings?
Well, you can do this already with ASAP Utilities, although you cannot find it in the description of the tools (yet).
First select the cells and then run the following utility: Numbers » Convert "textnumbers" to numbers
As a side effect (bonus) this utility will also make Excel recognize such empty cells properly again.