Board Game Arena

How does one use the Black Market? I had a Black Market occupied, yet when I went to build I was not giving the option of using a colonist, good or VP to reduce the cost of a building. Probably a user error... but I tried what I could to no avail. What is the process to activate the Black Market during the building phase?

As far as I remember, costs are reduced automatically. i.e. a building costing 4 would only take 3 coins from you.

I find it to be common, regardless of game or site, to have things like that happen automatically because the game knows to do it. That is assuming you don't have to pick between options.