Watercolor paper is made to take a little more abuse from a brush, and to hold water better than a regular cardstock can. If you're using a pretty heavy cardstock (110# or more) and not planning to do a lot of blending, you can get away with water blending. You can use a blender pen on regular smooth cardstock if you're not planning to do a lot of blending in any one place. Scrubby blending will cause your cardstock to pill.
Dina Kowal Creative ♥ Original Art Prints
If you hear a voice within you say you cannot paint,
then by all means paint, and that voice will be silenced.
Vincent Van Gogh