The Dravidians descend mostly from the first wave of people who left East Africa (Ethiopia, Somalia) around 65,000 to 70,000 years ago. Their maternal lineages belong to haplogroup M. Their original Y-DNA would have been C or D, but these lineages were later replaced by H and L. H might have evolved directly from CR in South Asia. L probably came from the Middle East or Iran.
They have dak skin because all humans had dark skin 70,000 years ago and the climate in India does not encourage a lightening of the skin. They look different from Ethiopians because they split from them so long ago (before Europeans split from Africans !).