Adam Clarke on Job 26:10

He hath compassed the waters with bounds - Perhaps this refers merely to the circle of the horizon, the line that terminates light and commences darkness, called here עד תכלית אור עם חשך ad tachlith or im chosech, "until the completion of light with darkness." Or, if we take תכלית tachlith here to be the same with תכלת techeleth, Exo 25:4, and elsewhere, which we translate blue, it may mean that sombre sky-blue appearance of the horizon at the time of twilight, i.e., between light and darkness; the line where the one is terminating and the other commencing. Or, He so circumscribes the waters, retaining them in their own place, that they shall not be able to overflow the earth until day and night, that is, time itself, come to an end.