@fify-- I don't think it's dangerous and sports players who use them do so under doctor control. They don't take random doses at will.
Even anxiety patients are prescribed beta blockers sometimes despite having regular blood pressure. So it's not shocking that sports players use beta blockers. If anxiety and stress is causing the individual to mess up their game, beta blockers can prevent that by keeping the player calm.
Also, as far as I know, beta blockers not only prevent high blood pressure, but they also prevent heart rate from falling too low. In a way, they protect from heart attacks.