It's kind of confusing that the example given for 真っ is the unusual reading of 真っ青... Also, is anything not listed just constructed as 真+[word]? If so, I feel like that should be indicated... and also, that would make 真っ二つ another confusing example...
Morphological rules like this one are often more straightforward to present in romaji or pseudo-IPA. Hiragana just obscures what’s happening phonetically.
This rule is also easier to understand as a two step process:
