Blessings

You've never used a blessing? wow, this is how they work, Once you put a blessing on an hero, they'll always gain double sp, meaning you can learn skills faster, and if you look at the top-left corner, you'll see two elements. (currently it's fire and earth) and if you have the legendary hero that corresponds with one of two elements, they'll gain the buff from the legendary hero if you go into battle with them, does this help? also, don't give hero's blessings unless you have the legendary hero too, unless youre desperate

They primarily increase certain stats (depending on the element of the blessing) when fighting alongside the correspondent legendary hero DURING the correspondent season. All of them give HP + 3. Earth also gives Atk + 2; Wind, Res + 4; Water, Spd + 3. Fire should give Def + 4, although I'd say Water and Wind should have worked in reverse, same with Fire and Earth.

Any blessing doubles gained SP of the blessed unit at any time, so there's that too. Also, it seems in March there'll be an event where only legendary and blessed heroes can participate (the new icon on the upper-left corner).

I would wait until they announce the new legendary heroes mode before you assign them, so you can see which heroes would most be useful for it.