I think the shubbies you had in there were big enough to breed.
They say shubbies are born black or brownish.
Maybe you have a combo of baby koi or baby Shubbies or koi shubbie mix. Anything is possible w/the fish you had in there.
I just read:
Are koi fish born black?
They can be pretty much any color that koi are typically known for (just not as defined as an adult koi), but they won’t be black. Some varieties of koi are bred to be all black, but unless you stock all black koi in your pond, it’s very unlikely to have all-black babies.