I wouldn't really suggest a bless. The powerful holy unit is more for adding a second magic type to the nation, although a very limited one.
That being said, if I am going for thematics, I will usually take an earth 4/6, nature 6 bless Earth Mother. Even with this bless, you have to shoot for Nature's Migration to make use of the Ancients, which is single player is easy enough.
In spring, the Ancients have a whopping 256 hp, so they make great use of the regeneration and reinvigoration from the bless.
Other bless options would probably be centered around what type of magic paths you want beyond nature.
Maybe in the next version I will add a sacred unit, or convert some of the existing units to sacreds, the Members of the Hunt may be good candidates.
The forest spider may be interesting as a sacred with #onebattlespell bless as well
