It's not the mid laner's blue. It's anyone on the team's blue. If you hesitate to take mage junglers unless you have
and don't have competition, well, three of those aren't exactly rare champions; but mids should be under just as much of an expectation to itemize for their mana issues (if they have them) as junglers are. More, actually, because
works better on them (and they can start with it) AND they get more gold and levels for free.
However, I certainly wouldn't complain if Riot increased the mana regen while fighting monsters for every jungle item from
onwards, and/or give it some kind of Essence Reaver effect vs. monsters only or refund a % of cost of any spell that hits a monster.
It might also be worth thinking about lowering Magus's CDR to 10 and putting some power into some other stat, though, since if you are building a major mana item in addition to your jungle item, several of the likely candidates like
give a lot of CDR already.
They probably don't want to put mana stats directly on Magus because it would feel bad for AP junglers without major mana problems (or who are manaless altogether) to be forced to take it. CDR is at least good when you get it, even if it may become wasted or redundant later.