It is typically blue with thin white shoulder straps, a white single vertical stripe on each side, and a presumptive tail hole as well as leg openings that stop all the way at mid thigh rather than at or near the groin folds (somewhat shorts-type).
The swimsuit's relatively modest design is presumably due to Cygames' internal policy not to overly sexualize or fetishize the horse girls. Despite the company's plea to do the same, fan artists of course have dressed the horse girls in skimpier swimsuits anyway, especially the traditional kinds of school swimsuits with leg openings that do stop at or near the groin folds. Eventually, the game's summer events Umamusume Summer Story and Super Smashing☆Summer Vacation feature several horse girls in more revealing swimsuits, although many of the swimsuits officially introduced in these events are not of the titillating kind.
Even if tagged as such in Pixiv and other sources, please do not add this tag on any posts of horse girls obviously wearing any other competition school swimsuit, or any other swimsuit for that matter. In fact, the 水着ウマ娘 (Mizugi Umamusume) tag in Pixiv refers to any swimsuit-clad horse girl in general, not necessarily one wearing this swimsuit.