According to manufacturer Faber, external blinds "are the most practical method of controlling solar heat gain. The problem of solar heat build-up is combated before it becomes a problem by mounting the blinds externally, where they intercept and defuse the suns rays. When Exterior Blinds are used in conjunction with air-conditioning, the air-conditioning units can be smaller, cost significantly less, and operate more economically because of the reduced demand on the air-condition system." ::Savannah Trims via ::Materialicious and the ::LEED Pro
The best way to deal with unwanted solar gain is to keep it out in the first place. One can do that with properly designed overhangs or bris soleil, which keep out the sun in summer but are designed to let it in during winter. However this is not very flexible. Another option is the exterior blind, quite common in Europe or Australia but expensive and hard to find in North America, where upfront cost always loses out to operating cost.