Bleaching can have unexpected effects. Black dyes have three typical shade types--green blacks, blue blacks, and brown blacks. sometimes it has to do with the material being dyed and sometimes it has to do with the chemical formula of the dye itself and sometimes it's a combination of the two. Personally, unless there's a reason to make the detail absolutely perfect, I'd rather get a fantastic effect in the spirit of the thing instead of correct details with average [or less] appearance. Is there a reason the coat can't be black? If it doesn't matter, don't sweat it this year, but put the coat in the sun next summer [leave it out from may until you need it again] and let the color degrade naturally. You won't get a brown, but you should get a neat distressed color out of it.
I know that when i accidentally got bleach on my black combat trousers they turned pink.
I also know that you can get clothes dyes that strip the colour out of your garment ready
for dyeing then use the colour you want to dye it next.
This is kinda a lame solution, but its an idea.
The Jeepers Creepers guy isn't exactly clean-cut, so maybe a messy solution could work for you. Just simply rub some brown powder into the material of the coat.
You could use cocoa powder (you'd smell delicious all night) or ladies' powdered foundation in a brown shade or even dirt could work. It would maybe even enhance the look and make it look more ragged and dirty.
Or for something more permanent how about spray paint? A thick coat wold give it a horrible crunchy, stiff texture, but a light dusting could be perfect. Just be sure to give it a few days to air out or you'll have a headache from the fumes!
Did some poking around and asked a friend in the textile dept of the UofMinnesota about bleaching and dyeing. She said bleaching is a bad idea unless you know exactly what dye and what material was used to make the coat --you could get some really awful [and not in a good Halloween way] results and end up losing what sounds like a cool coat. She suggested over dyeing the black with brown. It won't give you a warm choclate or even a bark brown, but she said it will make the black fabric reflect brownish highlights and if you redye it brown over a few years it will start to look more brown than black. And she thinks we're nuts because we'll put this much work into a Halloween prop.