What does Mellobloom's Thorny Blessing do?

Thorny Blessing is Mellobloom's core skill and costs 30 EP, so it competes with her other skills for the same energy. It does nothing on its own. The payoff only comes when someone on the team lands a critical hit with Grass damage, so its value depends on how much of your team hits as Grass and how often they crit.

That makes it a Grass team tool. A mixed team gets little from it, because only Grass crits spawn the thorns. In a full Grass line, such as the teams built around Irisalis where Mellobloom fills the support slot, every Grass attacker turns crits into extra damage.

What pairs well with it

Her ultimate, Rose Garden, puts a magic circle on the ground that gives allies inside it a 70% increase to Grass critical damage for 30 seconds. Thorny Blessing rewards Grass crits and Rose Garden makes those crits hit harder, so the two belong in the same window. The catch is that Irisalis also leans on ultimate charge, which limits how well the two share a team.

Crits also feed her rose mark. Her trait, Phantom Opening, adds Rose Mark stacks when a party member deals basic attack damage, and once a target has taken three critical hits the mark blooms and heals all nearby allies for 5% of her HP. A team that crits often gets both the thorn damage and the healing from the same hits.