You need a permanent ink that will not be dissolved by the watercolor or bleed along the paper fibers. I like the following two brands of artist quality felt-tipped pens:
(1) Faber Castell PITT artist pens. Four sizes: B, M, F and S (Brush, Medium, Fine and Superfine). Available in black, sepia, sanguine, grays and variety of colors and sets.
(2) Pigma Micron archival ink pens. Six sizes: 005 = 0.20 mm, 01 = 0.25 mm, 02 = 0.30 mm, 05 = 0.45 mm, and 08 = 0.50 mm. Available in black and a variety of colors.
Accent lines are most effective if used sparingly. For example, do not draw the entire length of a telephone wire or roof line, but rather hint at it with broken lines here or there.
As others have replied, also consider just using straight watercolors, with a rigor brush or liner brush, and a dark paint (such as a mixture of French Ultramarine Blue and Burnt Sienna). Or watercolor pencils, which can be used dry or wet, and on dry or wet paper, for a variety of effects.