Shouldn't LED blink 25 (30) times per second on AC?
I think you are confused by the sine wave thing. The voltage goes high 50 times a second and low 50 times a second. 



The LED is a diode, and basically half wave rectifies this AC signal. It will therefore only light during each half cycle. It will blink 50 times a second with almost a 50% duty cycle.



Interestingly, if you full wave rectify the AC, the LED will blink 100 times per second with a very short off time per half AC cycle.







simulate this circuit &ndash; Schematic created using CircuitLab