In just three lines, the speaker describes the act of drawing a line in ink and sets it against "the usual night." The line is small, but the speaker seems proud of it. The second line is the most interesting: "no paint so finely mixed and shaded." Paint can blend and shade into endless subtleties, yet the speaker claims that plain ink, used in one clean stroke, is finer than any of that. The poem seems to value simplicity and precision over richness.
The word "usual" matters too. Night here feels ordinary and expected, something that comes around again and again, maybe even a sense of dullness or darkness that the speaker is used to. Against that, the thin black line becomes a deliberate mark, a small act of making. Because ink is itself dark, there is a nice tension: the line is black on a background of black, or perhaps on the pale page, and either way it stands out through care rather than color. The mood is quiet, focused, and quietly defiant.