Save yourself some grief, and just use a few bags of a pre-packaged concrete product. I like Quikrete 5000 myself, mainly because it develops early strength and the coarse aggregate is small enough to fill most nooks and crannies. Do yourself a favor, and form and pour that open front corner with Quikrete as well. You should have "woven" the block courses at that location instead of leaving it open, but Quikrete will close things up nicely. Something the grout you are looking for wouldn't do, as it lacks the strength needed for that exposed location.