The blood both measures the ability of the horse to feel an "emotion" of any nature and its ability to transform it into a proper neuromuscular response. It is a heritable feature.
A horse 'near the blood" will perceive an external stimulus faster than a horse "missing blood", and faster it will implement the appropriate response: lift his foot to allow the groom cleaned it, lay ears to intimidate an intruder, making a swing to get away from danger, contract a muscle or bend faster and stronger joint to avoid an obstacle...
Blood is like money: "Cannot have too much so long as good use!"
Too much blood can make a horse too sensitive for an amateur rider and too little is incompatible with high-level sport.
The blood is not (yet) objectively measured and we propose to the breeder to evaluate "the blood impression". It evaluates daily on the mare and its production, as the case may be according to appropriate behavior at work, but also in the box and within the herd. It is to be assessed, for parameters of behavior at work, in relation to the state of health and fitness.
We do not have the same "blood" following a sleepless night or at the arrival of a marathon, than at the beginning of the evening or at the start of the race...
The speed of recovery, the speed a horse will get its condition and its weight of form back after a period of rest are also the evidence of the blood.