Filter a blank from a column

Try ISBLANK() instead of <> BLANK(). 

VAR _test =
ISBLANK(Contracten[Verwachte datum contract km bereikt])

Also, are you sure it is blank? You might want to read this article on blanks, nulls, empty (""), etc. 

EDIT: I didn't notice how you were using the variable later. I think az38 has nailed the issue.