In ChartDirector, everything that has a configurable color can be hidden by setting the color to transparent. For FinanceChart, the API to configure the legend box color is FinanceChart.setLegendStyle. In C#, it is like:
// Disable the legend box
c.setLegendStyle("normal", 8, Chart.Transparent, Chart.Transparent);
We use this API in the "Finance Chart Track Line" sample code to disable the default legend, so that we can add a dynamically generated legend that follows the mouse in its place.