If you are assuming that a black belt holder has advanced level skills in martial arts and he/she cannot be beaten by other practicing professionals, then it is not true. A black belt can also get beaten due to several reasons. Check out why black belts could also get defeated.
Non-serious about training
There are many people who learn martial arts as their hobby and do not take it that seriously as a way of self-defense. They might attend a few hours’ training at a martial arts class or practice on their own. Even if they get a black belt, they are not very serious about it. While other people may think them to be experts of the game, they really can be defeated by other serious students of martial arts.
Sometimes, learners are serious while their instructors are not that good in imparting the right skills to them. Some instructors are not even qualified enough to deliver the right art. There are certain instructors who are mostly focused on presenting martial arts as sports and encourage it for profit motives. In such cases, their black belts also do not meet that high level of skills that is generally expected of a black belt person.
Different judgment standards
Martial arts vary in forms. For example, some people go for Karate-Do and others go for Karate-Jutsu training. Now, the Jutsu training considers a self-defensive or protective point of view and is more valuable for a black belt person in Karate-Jutsu. On the other hand, a black belt holder in Karate-Do focuses more on the training as a sport. Thus, different judgment standards are applied to certify both black belt holders while their skills vary.
You cannot assume that a black belt person is perfect in protecting himself through martial arts. Black belts can also face a legitimate defeat from other professionals who are better.