Minecraft wiki entry for Development_versions, associated text: Blue variant of fire.
Fire lit on soul soil turns into soul fire regardless of dimension.
Burns forever, similar to regular fire on netherrack and magma blocks.
Generates naturally in the soul sand valley biome in the Nether.
Deals damage at a rate of 2 per second as opposed to the 1 per second of regular fire.
Does not extinguish when in the rain.