First of all I don't think it will lighten eventually, the aged brass will get darker overtime.

You could attempt to clean it with either a 0000 steel wool or the Brasso, but you should try on a conspicuous area first, the steel wool will reveal a lighter color but it might leave very minor scuffs and it will make it a little shiny (same with the Brasso) .

I design and make lighting fixtures and I darken the brass by hand, it is hard to control the darkening; if it is too dark, cleaning it might turn it into gun metal instead of a dark bronze.

Since he followed your orders he may have lacquered the finish to keep it that color. I have never heard of real brass getting lighter over time. By the way, when brass tarnishes it is not uniform and does look blotchy. I think that is why people like the antique brass finish, it is tarnish color but uniform in appearance.