Legend color in altair cannot be changed

Hello all,

I have a very simple question: In my barchart I add a mean ruler. I would like to have that rule in color black, and to appear in a legend. However, to get a legend for the ruler I have to specify a column name for the color property. Then it gets blue. How do I maintain the black line in both legend and chart for the ruler?

bars = alt.Chart(df).mark_bar().encode(

        ).properties(height=500, width=720)

        rule = alt.Chart(df).mark_rule(color='black', size=5, opacity=1).encode(