Don't compare all-blue T5 tanks with all-blue LED tanks. Blue T5 bulbs still have a good amount of red, yellow and green and just have a bit more blue for our eyes - they are still true full spectrum whereas the LEDs are not. Jason Fox also uses some Metal Halide and his "blue" bulbs are a lot like Blue Plus which are on the blue side of white.
The people who tried to light SPS tanks with ReefBrite-only and DIY all-blue setups had a lot of problems with monochrome color, lack of pop over time (color shifted), death and slower growth. Blue waves in that range are better at illuminating coral that got their color from a full-spectrum source... like Reefbrite XHO on when the people get home after some MH ran all day.
As a general guide, if you want good growth and color, then daylight for color rendering and nourishment and bluer (to your taste) for illumination.