Ol Doniyo Lengali is the only known carbonatite volcano still active on earth. Instead of being rich in silica like most lava, carbonatite lava is rich in sodium and potassium carbonates. As it cools the rock takes on a light grey colour. This lava much cooler than normal silicate lava, being about 500 - 600 degrees celcius (while silicate lava is about 800 - 1,250 celcius). Because of how low the temperature is the lava doesn't glow, giving it its characteristic black colour. This kind of lava is also much less viscous that your average water, having a consistency similar to water due to the lack of silica.
take a look at the flow