I'll just throw out rough numbers instead of digging up formulas because I don't want to butcher the math somehow, but I think I can explain it okay.
One point of MND adds about .5 HP cured to Cure IV before potency is figured in. That's far from exact, since the amount changes in accordance with soft caps and stuff, and generally it adds less than that if you have a ton of MND. That should give you some idea how small the effect is, though.
Magic Fruit and Peeonallofthem Embrace are more in line with the formula for Cure V, except starting with much lower base values and lower MP costs. So the effects of MND and even Healing Magic skill (stuff players can actually control) are very pronounced on these spells, compared to most cure spells which start with "large" base values and "large" MP costs.
Long story short, Blue Mage skips Cure IV and goes straight to weird Cure V's and that's why Blue Magic cures work the way they do.
