Will hanging black curtains substantially help heat my house?

probably not as much as good insulation to trap heat in…

no, unless it’s a small room w/ direct sunlight. But very heavy curtains will help, as well as stapling heavy duty plastic over the windows.

The colour of the curtains will have a lesser effect than the material and its thickness. If the curtains are too opague you will have less light and therefore a cooler room other than right near the curatin.

Keep in mind that the heat from the sun must be released in the room for it to be effective in heating the room. The previous poster suggesting a transparent barrier (storm windows) was on the right track. Closing the drapes, of any color, after the sun goes down will help trap the heat.