Highsmith outsources it.I'm not sure if Highsmith's does hot bluing or not. They may outsource it. But that's Greenfield area. Kind of east central.
About an hour west of there is Havlin's Firearm Finishing. I THINK they've done hot bluing in the past. I cannot find any current reference to it on their page right now - so I'm not sure. But it may be worth emailing him. https://www.indygunsmith.com/contact-us