We built a handheld device with only a few LEDs as a UI and found that people can easily tell solid from blinking and they can count groups of blinks up to maybe 3 or 4. Using blinking frequency did not work out well at all.

The WiFi LED was blue and there were lots of coverage problems at the customer sites . We wrote a blues song "The Blinking Blues" about it and performed it at our offsite. One of the lines was something like "I don't know if it's blinking fast or slow."