I play blast beats heel up. While I personally find heel up easier for extreme speeds, there will be other thoughts on this subject. Heel down can be just as fast (first 2 Origin albums, John Longstreth played heel down, has since converted to double stroke), and there is also slide and swivel techniques.
There are also numerous different types of blast beats. Single foot, double bass at the same speed as the hands, double bass at half the speed of the hands, 16ths on snare/quarters on hi hats/16ths with feet, gravity blast (one handed roll), 16ths with hands and patterns with the feet, it can be endless if you use your imagination. The main idea is the 16ths on the snare. Other than that there are no rules.