The issue is that the blue LEDs are very bright. You can use the same range as the other LEDs, but tend to overwhelm them if turned up all the way.
The library itself is an editable file. Open it up, and look at how it works. It uses C code very similar to that on the Arduino front end. ( Arduino/libraries/MeggyJr/MeggyJr.cpp and MeggyJr.h)
There are various ways that you might approach the issue, for example keeping the blue LEDs off in every other refresh cycle– if you did that, you would automatically dim them by half, increasing the useful resolution by a factor of two.