I refer to WINDOW, which is the free software that can calculate U-value and other factors for glazing including framing. I often use it for energy modelling in LEED submission. It can be downloaded here.

It has a library of shading material. The library has around 60 materials, and each material has Rsol1(Solar reflectance of the shade material, front side) and Rsol2(Solar reflectance of the shade material, back side). The value of Rsol1 and Rsol2 varies from 0.182 to 0.936. The capture below is part of the library that is sorted by color of material.

Rsol1 and Rsol2 in ProductName "Example" are 0.3. If you don't have any catalogue of specific blind product, you may use this value as Slat Diffuse Solar Reflectance. Other white materials with similar thickness has higher solar reflectance such as 0.5 and 0.7, but lower solar reflectance allows more solar radiation to stay indoors, so lower solar reflectance is safer side in terms of energy simulation.

The library doesn't have Solar Reflectance for Slat Beam, but normally the color of Slat Beam is same as Slat Diffuse, and Slat Beam doesn't affect much about energy simulation because it is only the bottom part of the blind. I think the same value as Slat Diffuse can be used for Slat Beam.

Another references are EnergyPlus ExampleFiles. Some files such as AirflowWindowsAndBetweenGlassBlinds.idf, EMSWindowShadeControl.idf and PurchAirWindowBlind.idf include WindowMaterial:Blind. Beam/Diffuse Solar Reflectance in all these files is set to 0.7. You can find which ExampleFile has what features in ExampleFiles.html (Normally, it's in C:\EnergyPlusVx-x-x\ExampleFiles in your PC). It's useful.