Would blocking sunlight on Venus help make it habitable? Lets say we had the technology to create a mini-Dyson structure, like a shutter style curtain between the Sun and Venus, blocking the sunlight in a controllable manner to cool down it's temperature. 
Could it be the first step to reverse the greenhouse effect and make Venus more "terraformable"? Or at least more explorable like Mars? Would there be any unintended consequences?
Would this be useful on other hot extra-solar earth-like planets?
The fundamental problem is the heat capacity of the planet. If you keep it out of sunlight long enough it will eventually cool to the temperature of the background ratiation, bit it will take a long time. If we just treat it as a blackbody surrounded by a 0 K universe the cooling time will roughly be $t_{cool}\approx 2.5\cdot 10^{11}$ seconds if we want to reach 300 K. That is about 7900 years. Wait for about 29,000 years and you can start scooping up frozen carbon dioxide. And this assumes good heat conduction and that the interior is not hotter - I think there will be bad geological issues when trying to change the crust temperature.

Another issue is of course the total lack of sunlight. Martyn Fogg describes a neat solution in his book Terraforming: orbit a reflector in a polar orbit, angled at 45 degrees so it reflects sunlight to the planet. This would for the right orbit give a daily cycle despite Venus rotation issues and the shade shield. 

Whether this is a sensible start for terraforming or just evidence that the technology does not help very much depends on your level of optimism and timescales. For exploration you may want a method that does not transform your exploration target as much.