I didn't want to clog up the old thread but blue boosting is where you hold the slide and then straighten up.. getting a boost out of the back of your kart.

See

www.mariokart64.com/mksc - then click on the top banner "driving tips" then read that.

Its main use is parallel with NBT (New Boost Technique) on SMK. If you can "hold" a boost by driving sideways etc then you can manipulate the right time to release it.

The best time being when you want to cut some grass/dirt/sand which you'd have to go around. Finding the shortest distance from A to B.

I'm sure term will be able to provide a good video to show this technique. (I only use it on smk tracks at the moment though till i re-learn all of the old ones).