I'm not so sure this is as balanced as you think. Look at growth creatures. Let's take lava destroyer.
5 fire, 7/1, +2/+2 for 1 earth
Now yours...
6 light, 4/4, +3/+3 for 2 light on a creature of your choice
Lava destroyer's ability costs less overall quantum, but at the same time priest's gives a bigger boost. However, note that lava destroyer's ability costs a different element from its casting cost. Overall, the lava destroyer's ability cost is harder to pay.
Next let's look at the ability itself. I've already explained that the lava destroyer's ability is harder to pay so it can be said it costs more, but +3/+3 on a creature of your choice is much better than +2/+2 on just the creature using the ability. So priest's ability costs less and has a better effect than an already strong lava destroyer.
Then let's look at the starting stats. Lava destroyer's main weakness is it's ability to be easily killed/eaten in the early going, since it is 7/1. Its +3 starting attack (when compared to priest) does not balance the -3 defense. If lava destroyer started 4/4, like priest, I would like it much better than its current 7/1 even though it would be less useful in speed decks. With 4 defense, the priest can survive most spells and after a single turn is out of range of an elite oty's eating ability, while a lava destroyer must wait 2 turns (3 turns if the oty has devoured any one thing). On top of that, the priest's element also has the card blessing and can be used with it to get a 7/7 creature right on the turn its played, making it very hard to kill before it gets going.
Finally let's look at the casting cost. Priest costs one measly quantum more than the lava destroyer in an element where high tower counts (and therefore lots of quantum) is beneficial (miracle). Though fire has fire bolt and fahrenheit, most decks that use lava destroyers don't use either. So for most light decks, 6 light isn't actually all that much.
So since I feel like I wrote an essay, tl;dr version:
-Blessing on a stick is not a good idea (unless it costs a LOT, like a light nymph, for instance. With an ability cost of 4 or so upgraded)
-Compared to lava destroyer, priest's ability cost is easier to pay and also is MUCH stronger
-Priest's starting stats are better for its purpose than lava destroyer's since it can withstand some blows when its at its weakest
-The priest would be in an element where having lots of towers is beneficial so its +1 casting cost is negligible
I wouldn't mind blessing on a stick, I just think it would need to cost a lot more than your's.