Kind of like a visual form of Braille or something. If the blind can somehow interpret and make sense of different arrangements of bumps on a wall, then surely the TOS crew could do something equally impressive with lights in the 23rd century.
Visual braille - I love it!
Gotta say, though, I think the advanced training to interpret the lights is a hurdle for me. I can't think of a reasonable way to argue that interpreting combinations of blinking lights is more efficient than a visual readout. It's like arguing that Morse code is a more efficient mode of communicating audio than voice.