Erm, it's not like anyone is suggesting "block all sunlight". We're talking about blocking at most a few percent.

On this topic, I've always wondered if you could make such a solar shade that hovers just on the far side of the Lagrange point between sun and earth, balancing radiation pressure and gravity. Does anyone know if that would be passively stable?

Not quite, something would have be done about lateral motion. L1-L3 are only stable along the axis between the orbiting bodies. L4 and L5 are local gravitational minima, but that doesn't help us here.

Here you go:

https://en.wikipedia.org/wiki/Space_sunshade

https://news.ycombinator.com/item?id=10209794

Edit to answer your question: No, it wouldn't be passively stable. But it could be actively stable with solar-powered motors to move mirrors, so it wouldn't need fuel.

Not in the deserts or the oceans, we don't.

We need light on the oceans for phytoplankton that release oxygen.