For every blank cell , sum those above it

I would do like this with VBA

Sub Testing()
Dim rng As Range
Dim r1  As Long
Dim r2  As Long
r1 = 1    ' this must be the first row with value>0
For Each rng In Range("C" & r1 & :C28")
    If rng.Value = 0 Then
        r2 = rng.Row - 1
        rng.Formula = "=SUM(R" & r1 & "C:R" & r2 & "C)"
        rng.Font.Bold = True
        r1 = rng.Offset(1, 0).Row
    End If
Next rng
End Sub

The C28 you must change with a lastrow value + 1, the lastrow must always be a blank one.