The 21st Washington International Horse Show opened last night at Capital Centre with the usual international flourish, but a snappy little mare from the western plains stole the evening by winning the first blue extravaganza.

Miss Kay Bar, owned by Gwen Belyea and ridden by Jon Hart, won the initial battle among the western pleasure horses by outscoring Lynn Little's 9-year-old chestnut gelding Port. The winner, a 5-year old chestnut, shows out of Laurel. Third place went to Miss Leota, owned and ridden by Kevin Joyce of Davidsonville, Md.

The western horse division included entries of any breed or combination of breeds. Among those most prominent are the appaloosa, Arabian, and quarterhorse plus Morgans, pintos and crossbreeds.

Sally Perry's Quest for Cajun turned in the best showing in another western pleasure class, winning the "Lady to Ride" class over Cindy Helwig on Briomen. The winner, a 9-year-old, shows out of Queen Anne, Md.

One of the wildest events on the western circuit is the musical tires class and opening day's event was no exception. The eventual winner of the blue ribbon was Joanne Walsh's Stower Miss, an aged chestnut carrying the colors of its Edgewater, Md., stable. Ridden by Cliff Sweeney, Shower Miss outdistanced Kelly Neighbors' King Bar Cat with Joseph Cage's Ma Mere third.

The hunters begin their week-long quest for Washington International titles early this morning, the green hunters competing for the Robotyping and Fairfax Hunt Race Association challenge trophies. Centered in the Maryland and Virginia horse county, these powerful and graceful horses have always held a special place at the show, beginning with Morton Smith's 1958 winner Witchcraft and going through Jane Womble's grand titlist of last year, Henry the Hawk.

A new scoring system for hunter competition will add to spectactor interest during the evening classes. It is the open numerical system, in which each of the three judges gives a score after each round, with the score flashed on the Tel Screen immediately. This will allow spectators to follow the scoring from start to finish, much like a diving meet.

Among the top hunters showing this week will be the newest conformation sensation, Early Light. The 5-year-old chestnut gelding, competing in the green group, was the national champion last year and will be one of a string of entries from Cismont Manor Farm in Keswick, Va. Another Cismont entry, Mr. and Mrs. August Busch's Stocking Stuffer, was grand hunter champion here in 1975. Both horses are trained by Kenneth Wheeler and will be handled by the Steve Cauthen of the show ring, Charlie Weaver of Fairfax.

The Inverness Farm Challenge Trophy, won last year by The Cardinal, will top this evening program, which opens at 7:30. America's top amateur and professional jump riders will go against members of the Belgium, Canadian, Irish and United States national teams in this international-open jumping. Two world-class foreign riders. Graeme Thomas of New Zealand and Juan Rieckehoff of Puerto Rico, are on hand for the eightday battle.

The program will open with a regular hunter scramble for the coveted second Marlborough Hunt Challenge Trophy and will close with the driving class pairs. The latter, a part of the Washington International's newest division, is a class designed to test the ability of the whip.