I have “Highlight blocks when mouse hovers in gutter” checked in my editor preferences. However, I don‘t see any block shading when my mouse is in the gutter — just the code folding arrow markers.
Just retested here on 10.11:
I have stock background colors, but I tried it with a light blue background and it worked there as well. Perhaps show us your prefs.
I guess I misunderstood — i thought it would be the code text that would be shaded, not the margin indicators, as if I had done a series of Balance operations to move up the code structure, but with a different shading than the selection.