The structure that determines a skill's requirements is at 0x6900a. It's in order of the skills by ID value, naturally, and goes as follows:

1 byte Level
2 bytes HP
2 bytes MP
2 bytes Attack
2 bytes Defense
2 bytes Agility
2 bytes Intelligence
1 byte Prerequisite skill
1 byte Combine skill 1
1 byte Combine skill 2
1 byte Combine skill 3
1 byte Combine skill 4

Blazemost would have BlazeMore as its prerequisite skill, in case that byte needed any clarification. That's the skill that will be auto-replaced with the new skill once you meet the new skill's requirements.

The combine skills are a list of up to four skills that, if a monster knows them all, will allow that monster to learn the skill automatically when it levels. Something along the lines of "derived <skill> from the skills it already knew" will alert you when this happens.

Note that MegaMagic is a special case. All five slots for MegaMagic are filled and the byte that would normally be used exclusively for a prerequisite skill is instead used as a fifth combine skill.