As per previous comments, ISBLANK will return FALSE even if your formula in cell C1 returns an empty string (""). ISBLANK will return TRUE only when a given cell is "truly" blank, i.e. does not contain any formulas or values.
As an alternative, try the following functions:
=C1="" <- will return TRUE, assuming that formula in C1 returns an empty string ("")
=OR(ISBLANK(C1),C1="") <- both formulas combined; will also work in case you remove your original formula from cell C1
=NOT(ISNUMBER(C1)) <- will return TRUE in case the result of your numeric formula is empty (e.g. =IF(LEN(A1),A1+B1,""))
One more formula that you may find useful:
=ISFORMULA(C1)
Hope it helps.