How to loosen/soften a leather belt?

That method will surely change the appearance. I would call a tack shop, and ask them what they do to treat harnesses, reins, saddles, etc.

I just made a belt where the keeper thing was made by cutting slits in the leather and twisting it. This caused it to be too tight and the belt didn’t slide through easily. I sprayed it lightly with water and then worked and stretched it while it dried. Worked fine. Since it was already dyed a fairly dark color, it didn’t change the appearance.

I always used saddle soap on a new baseball glove to soften it.

Most anything you use will darken the leather though.

If I just keep using it, will it eventually wear in a bit?

I’d use leather conditioner or saddle oil. Both work very well.

Neatsfoot oil and saddle soap. Let the oil soak in from the skin side til the suede side turns, Then Saddle soap to keep the oil from soiling your trousers. Then wear it a lot. Usually does the job for me.

Only use a product that is specifically designed to soften leather. Saddle soap, or “glove” softener (baseball gloves) are two of the ones I know that do a great job. You probably would get some good information from a leather store in your area.

Neatsfoot oil for all your leather needs :)

Vasiline is very good for spftening leather, but I spotted a shoe leather softener in Clarks shoe shop yesterday, that may work?