Tiny blood clots running vertically under the nails, and are associated with, inter alia, infective endocarditis, and trauma. At first, they are plumb colored, but darken to brown or black in a couple of days.

In cardio-related, clots can migrate from the affected heart valve, and find their way into various parts of the body, including the finger, which damages the capillaries, resulting in a splinter hemorrhage.

Source: http://www.dermaamin.com/site/images/clinical-pic/s/splinter-hemorrhages/splinter-hemorrhages7.jpg