The colour of the black beaches (such as Kamari, Perissa and Perivolos) is obviously due to the volcano. The lava which covered the island resulted in the island's beach colour. The black sand can be scorching hot in summer and somehow darkens the clear waters, but because of the weight, it does not stick to the body as the golden sand does!
The Red and White beach are equally impressive, since they were also created by the volcano. Enjoy your swim!