It depends on how much layering ON the surface you plan to do. If you are going to use the pens for simple one pen blends then any smooth finish card stock should do. 

If you plan on laying down multiple layers of color on the card and really working the blend on the surface then I have found working with card stock designed for alcohol pens is best.

We like Neenah Classic Crest Solar White, Bazzil’s Smoothies and Simply Smooth, Copic's Bending Card X-Press it but just about any card stock that is designed for use with alcohol pens works.