The LEDs blink in accordance to the frequency in use.
When the frequency is fast enough, the LEDs blink so fast that they appear to us as being always on.
Much like your TV, which is also blinking 30 or more times a second, yet you perceive a smooth solid moving image.
When you get to very very high frequencies, the LEDs will be turning on and off so fast that they actually will look dimmer, as they never get a chance to fully illuminate.
Also you can read this blog that explains how LEDs works:
For more details, please check the link: