Well, your solution to wanting an “unobtrusive” edge is just the sort of thing master embroiderers do. So you’re not an idiot after all. If you dislike plain blanket stitch, you can always do a knotted edge, picot edge and any number of other decorative ways to finish. But rest assured, plain closed blanket is centuries old – and it works. One thing that makes it very nice is by making running stitches. You could put those running stitches on that pulled thread line. Also, a running stitch on the inside edge of the area to be blanket stitched. This gives a rounder appearance and ALSO helps to hold things together better. Meredith used a satin stitch.
The only thing I can think of off the top of my head is to perform a subsequent band of blanket stitch (with the running stitches) around the first satin stitch. Maybe a tiny space between the two areas into which you could embroider a feather stitch, coral stitch, tiny herringbone. I’m trying to envision piercing the satin stitch and making some fancy buttonhole loops. Not sure it would work – and depending upon the ground, might weaken the edge. Whatever you do – the looseness of the ground (tightly or loosely woven) will determine the outcome. Do I understand that you did this diagonal stitching in a smooth run – as averse to something akin to Hardanger with little steps.
If you’re going to cut out this piece so that it is a diamond – and you want it to stay intact, you’re gonna have to bite the bullet and blanket stitch it following the diagonal – not with all stitches up/down. If you’ve done much Hardanger, you know that even THAT can be a dilemma on the edge (the plain satin stitching) and machine stitching first – or hand running stitches – are often employed. I’ve read quite a few posts on this edge dilemma (I’ve done only one small Hardanger piece). The loop of the blanket stitch holds the ground threads so that they can’t move and covers the edge at the same time. It prevents ravelling. Plain satin stitch won’t work – nor will herringbone – even closed herringbone. I’ve tried plain satin stitch for cut out sections on fine cloth. It doesn’t work. And running stitches first help stabilize it all. I’ve done this both ways – and know that it works better (the running stitches). That’s why all cutwork embroidery – of any ethnic origin that I know of – uses blanket stitch or knotted buttonhole stitch.