
To define how good this card can be. Take note of Favor of the Sun. You can trigger Favor of the Sun after you've seen the first bless token in your draw and thus guarantee Blessing of Isis triggers right there. It's a strong combo, but I'm not sure Blessing of Isis is worth it by itself.
The probability to see two bless tokens goes from neglible at 2-3 tokens in the bag, to nearly 20-30% at the maximum 10 tokens, with a Mary doing her very best to fill the bag with goodness, and/or other characters helping out somehow, those odds can actually be rather good and consistent, netting perhaps 3-5 extra triggers. The trick then is to take characters who've got great abilities, Leo Anderson, Nathaniel Cho and Sister Mary herself all like this effect.
As a last method to support this card, there's seal mechanics. When you've got a Chthonian Stone, the Day of Reckoning weakness, Seal of the Seventh Sign and other mechanics to block out other tokens, this in turn enhances your chances at seeing back to back tokens, this is actually a reciprocal synergy so a large group playing into this theme together can set up games where a heap of negative tokens is locked out entirely and blesses are more then half the bag.
I dont think Blessing of Isis is a particularly excellent card, for the simple reason that it relies on setup and randomness, but it's fun and can be extremely powerful in that niche window when you're consistently seeing those lucky draws. At least once you've set up, the that trigger Blessing of Isis do go back into the bag when triggered, so Blessing of Isis is a bit of a "self refueling engine".