If your mare is homozygous agouti (AA), you will never get a black foal from her, only bay/brown when bred to homozygous black. If your mare is heterozygous agouti (Aa), you have a 50% chance of getting a black foal. Breeding to a homozygous tobiano or splash stallion guarantees the passing of that gene but not how much or little it will be exhibited. You could get a solid foal or a very loudly marked foal, just a roll of the dice.
But before you consider the color of foal you could get, consider the actual value of the breeding. Stallion quality and mare quality. Your mare isn't registered, the best you could do is have a foal registered in PtHA. Every confirmation fault of both mare and stallion has to be weighed and measured. You also need to know if either carry HYPP, GBED, and other genetic diseases common in the stock breeds. You also need to know the frame status of both stallion and mare, because if both carry frame every wise breeder will not risk a 25% chance that their foal will be dead 3 days after being born.
Breeding is expensive, a gamble on what you get, risks the life of both mare and foal because when things go wrong, they go very wrong. Basically, when breeding, color is the last consideration and icing on the top if you get it on a well put together foal. Weanlings are hitting the market right now, you can shop by color and gender to find your dream baby with the color you want and find one that suits your needs. ;)
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