isblank() is a function that takes one argument. so your use of isblank is incorrect because there is no argument. To get the count of blank cells in a column you need to know two things:
1) how many cells are in the column and
2) how many cells have something in them
you can the the count of cells in a column using the function rows()-- no arguments necessary but you can provide a cell or column if you want.
you can get the count of cells with something in them using the function countif()
you can combine like this:
A note... B3 should be left blank for this example.
There is a free, downloadable USers' Guide and a function reference you may find helpful: