At the top of the site, look at the top right corner next to the Logout link, theres a bell for notifications, and a magnifying glass. The magnifying glass allows you to search every single horse in the game, regardless of whether they're for sale or stu/dbrood.
I've just spent an hour searching, using variations on โblueโ and also โgallopingโ in the name search, looking for Leopard and KIT blue roans, looking at thoroughbreds. A blue roan thoroughbred colt named โBlue Runโ turned up and I freaked out thinking I'd found it hahaha but nope.
Absolutely no idea! Hope someone finds it.