Middle Grade Horse Book Series

I’ve been trying to remember the name of this series for the life of me and I can’t. The characters were around middle school/early high school age and the main setting was the stable they rode their horses at. I can only remember the plot of one of the books in the series. In it a famous race/show horse goes missing and it’s believed that the groom stole it. The main character comes across him and the horse in an abandoned shack nearby. The groom stole the horse because someone(I think the barn manager) was poisoning it so it would lose and upcoming race. And so the two of them work together to reveal the plot and save the horse. This specific book can’t have been written later than 2009(others in the series may have been written after). It was likely published by scholastic because I remember getting it at a school book fair. I remember the cover was solid blue with a black horse and that the rest of the series had similar covers(solid color with a horse on them).
I know for sure that it’s not the Thoroughbred, Phantom Stallion, Pony Pals, or Saddle Club series. It’s also not from A to Z Mysteries or Animal Ark. The series was entirely about horses.

This sounds like Horse in Danger from the Sandy Lane Stables series.