My answer below assumes that you are hoping that a 0.5 value for the blinds will result in a 50% reduction in solar gain through the window. In other words, you are not evaluating more complex interactions between the blind and window, like its effect on radiation heat transfer.

With the current version of E+, the simplest way to model this effect is to add a shading surface (Shading:Building:Detailed) the same size as your window and slightly offset from it. You can then set a transmittance schedule for the shading surface. (Note that the shading surface impacts direct/diffuse solar from the sky but does not affect solar from ground diffuse, so you will also need to adjust the window's view factor to ground input. If it's a vertical window, the default view factor to ground will be 0.5, so you would change it to be 0.5 * shading fraction.)

There is also a new feature proposed for the next version of EnergyPlus to provide a simplified shading object that will make the input into EnergyPlus much simpler.