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||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.