Blenheim Market Harborough Black. The Blenheim Market Harborough is made from the best quality leather available to give to a quality training aid which will help provide more control. The Blenheim Market Harborough is supplied with continental web reins and is best used during ridden flat work. The Blenheim Market Harborough is designed to provide pressure to the bit if the horse raises its head too far, allowing you to gain more control superior to that of a running martingale. The Blenheim Market Harborough is also excellent at building muscles for an improved top line.