It's been proposed to put optical elements in space to reduce sunlight coming to the earth: Realistic cost and feasibility of sun-shading?
That question states that the studies have all hypothesized putting these things at L1. Why? L1 is quite far from Earth, so very little of the sunlight that passes through L1 would ever have gotten to Earth anyway. Wouldn't you get more bang for the buck by putting things in low earth orbit?
[EDIT] The source of my confusion was that I'd been thinking of L1 as the earth-moon L1 point. It hadn't occurred to me that they were talking about the earth-sun L1 point.