Don't mistake Ijen's blue fire for lava. The blue glow is actually the light from burning of sulphuric gases. The gases emerge from cracks in the volcano at high pressure and temperature up to 600°C. When they come in contact with the air, they ignite, and sending flames up to 5 meters high. Some of the gases condense into liquid sulphur, which continues to burn as it flows down the slopes. The burning happens day and night, but it's visible only in darkness.
Blue volcanic fire was described in antiquity in Italy on the south slope of Mount Vesuvius and on the island of Vulcano. Blue flames may also be observed at the base of the plume of erupting volcanoes, when ash explosions occur. In Ethiopia's Danakil Depression, the sulphur dust in the soil of a hydrothermal vent ignites to form blue flames as well. One can see similar "rivers" in Yellowstone National Park in the United Stated of America. Forest fires were the cause. Heat from the blazes melted the sulphur around hydrothermal vents. Once in Yellowstone one can see their traces as black lines (source: http://news.nationalgeographic.com/news/2014/01/140130-kawah-ijen-blue-flame-volcanoes-sulfur-indonesia-pictures/).
The largest blue flame area in the world
Ijen crater lake is the world's largest such body of water filled with hydrochloric acid. It appears to be relatively common to find molten sulphur around volcanic hot vents, as the mineral has a relatively low melting point of 115°C. Temperatures at hot vents often exceeds that (source: http://news.nationalgeographic.com/news/2014/01/140130-kawah-ijen-blue-flame-volcanoes-sulfur-indonesia-pictures/).