I'm trying to breed a black chestnut with zero success. Does anyone have any advice?

There's no way to guarantee black chestnut as it's a random and very rare variant of chestnut. For some perspective, I've been breeding hundreds of colors foals each year with chestnut being my primary base color, and I've never gotten one. It's all luck.

Chestnut horses must be recessive for Agouti (aa) in order to have a chance at breeding/rerolling for Black Chestnut

bringing this back up, I have this horse that is aa and chestnut so is there a possibility to reroll to black chestnut then? And also black chestnut jus to clarify has to be chestnut with a homozygous aa correct for a chance at it?

Yep they must be ee aa

You can possibly reroll them to black chestnut but honestly I wouldn't waste your rerolls unless you are rich lol. Many many people have wasted hundreds of rerolls trying to get a black chestnut with no success. It's just way too rare. Unless you get extremely lucky