Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
    Dim CellToCheck As Range
    Dim wf As WorksheetFunction

    Set CellToCheck = Sheets("Бланк КЛ").Range("B8")
    Set wf = Application.WorksheetFunction

    If wf.CountBlank(CellToCheck) = 1 Then
        Cancel = False
        Exit Sub
    Else
        MsgBox "Текст"
        Cancel = True
    End If
End Sub