One Surprise Chestnut Colt?

[BlackOak2]

In reference to your edit, have you ever used the Punnett Square? This will help to identify how many likely offspring will result from a set of parents. Since both parents are hetro for black, they have a 50% to throw single blacks, 25% to throw double blacks and 25% to throw chestnuts. It can be looked at as 75% black-based foals and 25% chestnut.

Oh sorry, I worded that wrong haha. I was double-checking if both the parents turned out to be Z/z, the foals could either be Z/Z, Z/z, or z/z? I was trying to figure out how I could know if some foals were Z/Z (I guess by breeding them to non-chestnut/agouti horses?)

Like I said above, you can confirm a horse is hetro- (Z/z) when a non- pops out with a non- parent, but there is no real way (until gene-testing comes into the game) to see if the horse is homo- or not.
However, with enough generations that test and continue to pop 'true' (continue to pop nothing but silvers), you can theorize with fair accuracy that you have homozygous silvers.