Basically in a normal flame you have hot combustion products which are less dense than the surrounding air so buoyancy makes them rapidly move upwards. As this is happening they are cooling down and there isn’t enough time to complete combustion so soot is formed. Blackbody radiation from the soot is the characteristic orange part of the flame that we are used to seeing.
In microgravity there is no buoyancy-induced convection so what you see is a pure diffusion flame. That means that there is a thin interface in a sphere around the vaporized fuel stream where the fuel and oxidizer is perfectly mixed to make combustion take place. the fuel burns nearly completely without being pulled away by buoyancy effects, thus you just see a sphere of perfect blue flame.