[aerohead]

Cal Tech, FoMoCo, and others used a mixture of lamp-black and kerosene back in the 1950s - early 1960s.
Online, there's an iconic photograph from Cal Tech's wind tunnel, where the 1963 Chevrolet Corvette Sting Ray is undergoing this form of flow visualization.
The trick is to nail the proper viscosity of this mixture, so it won't just run all over the body before the tunnel gets up to supercritical Reynolds number and images can be captured.
And this technique will not reveal vorticity. Only tuft-screens or smoke will do that.