It depends on the horse's genetics.
Chestnut is recessive to black, therefore a chestnut horse
always carries two chestnut genes but a black horse could be
carrying two black genes OR a black and a chestnut, but the black
is dominant therefore the horse has a black coat.
Think of it as the black coat always sits 'on top' of the
chestnut coat so if a horse has a black gene it will always show up
but chestnut will only show up if there are no black genes at
all.
If you breed a homozygous dominant (two black genes) black horse
to a chestnut, the foal will always be heterozygous dominant black.
(One black gene and one chestnut gene)
If you breed a heterozygous dominant (one black one chestnut)
black horse to a chestnut, the foal has a 50% chance of being
heterozygous dominant black and a 50% chance of being homozygous
recessive chestnut. (two red genes)
If you have no idea what your black horse's color genes are,
think of it as having a 75% chance of a black foal, 25% chance of a
chestnut foal.
