Turning Cursed status on a character directly into Blessed status would be fine (being cursed isn't that bad anyway), but I wonder if turning a cursed surface into a blessed one might be a little too OP.
Cursed fire can be annoying, but if you can just transform it directly into blessed fire, it starts to heal you and makes you immune to fire for 2 turns. Maybe that's a little much for a single Bless spell.
Btw, there is already a way to turn cursed fire into blessed fire directly using just 1AP. Toss a blessed oil flask into the fire. You can also use blessed water balloons and blessed water arrows to turn it into healing steam clouds.
I would actually suggest to Larian to make crafting those items just a little easier because all of them require Source orbs, which are very rare especially in the first half of the game.
But you don't even have to remove cursed fire. Once you get to Reapers Coast, ingredients become abundant, so you can start crafting those resistance potions. E.g. drink a medium Fire resistance potion (50% resistance) on a character who has the Five-star Diner talent (a must for my warriors), it doubles the effect for 100+% fire resistance for 3 turns. So instead of keeping your physical tank builds away from fire cuz they have low magic armor, you send them right in the middle of the necro hell to bash some heads open, while they heal in it, (all the while your battle mage rains more hell on top of them for even more healing and more barbecued enemies).