I disagree that it's a case of flavor. There needs to be a marked difference between arcane and divine magic. Just as there is in vanilla D&D.
As of this apparent major change in Bless land they just moved one step closer to each other. Both are now able to boost troops, cast protective magic, affects prosperity in the same way, summon troops (with a wider array of options for arcane), cause plague and warding against evil/chaos/etc.
Divine has a couple of specialities, like lowering upkeep, increasing province size, investiture, protecting a mundane holding (whereas arcane can protect source holdings), affecting guilds and trade and of course healing.
But arcane has a long list of special abilities; ward whole provinces, turn armies invisible, paralyze an army, subvert an asset, greatly increase or lower income, throw up a castle in a blink of an eye or tear a fort apart, cover lands in a blanket of winter, teleporting units across vast distances and obviously a variety of kaboom magic.
I'd think it logical that divine and arcane magic would affect prosperity in different ways.
Just like vanilla D&D has divine magic giving blessings/curses, an outside influence affecting the individual's prowess, but not taking direct command. Whereas arcane has more of a controlling/directly affecting aspect, people grow to double their size, turn invisible or the mind is "fucked" as you say.
This was exemplified by bless/blight land giving a one-time bonus/penalty to prosperity, increasing "base prosperity" so to speak, whereas demagogue boosted prosperity up/down for as long as it worked, not affecting base prosperity.
And now they're doing the same? I'd much rather prefer making bless land a 2nd level spell or give demagogue a duration of more than 1 turn, than let divine and arcane magic be able to do more of the same.