The blm will have the naturaly mana reg to, like every other class has it.
It is normaly blocked in the fire phase and even the regeneration buff skill dont work in the fire phase.

There shouldnt be any blocking in the ice phase. It was only increased up until now. What gave the random high regeneration in that phase.
It is now simply be changed, that ice attacks will now have a fixed number, how much it will regenerate the mp again. Probably similary to some skill of the Paladin or DRK.
Your surrgestion sound simply like the system whe have yet.
And i dont even see any problem there. In the Trailer was the ice phase keeped very short to. I think after 2 ice attacks had he switched back to fire. More or less the same number, whe need now for the ice phase (sometimes 1-2 attacks more).