Modulating output is different legal territory from flashing/blinking. If you're talking about a modest flashing boost to a steady headlight, that's used on many motorcycles to increase conspicuity. Many states allow modulated headlights, subject to regulations that usually limit the amount of fluctuation.

(On motorcycles, headlight modulation was originally a defect, caused by the varying voltage of magneto-driven electrical systems. Today, with alternators and larger batteries, motorcycles maintain steady voltage, just like cars, so they add circuitry to intentionally vary the headlight intensity.)