How to earn more blessing items?

Hope for a quest that gives them.

Summoning legendary heroes gives you a blessing based on what type they are - otherwise, you are at the mercy of IS releasing quests or new blessing maps.

I think the Forging Bonds event starting tomorrow/tonight will have them available as rewards!

You can get one blessing from rival domains each week, one blessing is usually included as a TT reward and then the rest comes from quests. The other way to get blessing is to summon legendary heroes.