Parallel streamers of clouds across the sky which seems to be at mid-level of the atmosphere. The cloud`s dark colour is accentuated not because it is thick but perhaps its composition of tiny droplets which block more light than larger cloud droplets. Furthermore, the clouds was on the eastern side of a darkening sky at sunset hence making it look darker than it actually was. The weather situation is explained here. This cloud was producing at least some rain in its base as evidenced in the photo of the first thumbnail. Cold air pool was present at all levels of the atmosphere rendering it somewhat unstable.