Whisper White does work great - any smooth finish non glossy does. The ticket for getting smooth blending is to use NO pressure on the brayer at all. I took a class from Michelle Zindorf and that tip has changed my relationship with my brayer!
Barely hold the handle and just steer it without pressing. Go over it several times to get a gradient.
I don't use glossy cardstock for exactly this reason, and her technique works beautifully on smooth white cardstock.