Magnetic fields are widely used to brighten spin-forbidden dark excitons, but the inverse effect on an optically bright exciton has remained largely unexplored. We demonstrate the complementary phenomenon of magnetic-field-induced darkening of a bright exciton in Mo_(0.68)W_(0.32)Se₂. At 4.2 K, an in-plane magnetic field up to 25 T activates a dark-exciton line 14.2±1.3 meV below the bright exciton while suppressing the bright-exciton emission by about 85%. A bright-dark mixing model quantitatively captures the coupled energy and intensity evolution with an effective in-plane g factor of |g_∥|=5.2±0.2. Alloying is crucial to the effect, as it brings the system close to the bright-dark crossover, where a small bright-dark splitting is combined with unusually strong in-plane magnetic coupling. This places the system in a regime where field-induced level repulsion raises the bright-like branch and strongly depletes its thermal population. These results identify a general route to magnetic control of exciton populations through engineering of the bright-dark hierarchy.