What kind of shrub has dark purple berries in pairs surrounded by reddish petals?

The shrub you are describing is likely the Elderberry (Sambucus), particularly the European elderberry (Sambucus nigra). It produces dark purple or black berries that often grow in clusters, and when in bloom, the flowers can have reddish or pinkish hues. These berries are edible when cooked but can be toxic when raw, so they should be handled with care.