From the manufacturer:
This rigid, warp-resistant foam board with its smooth, white, clay-coated surface is ideal for mounting, paints, markers or screen printing. When cut, the edge bounces back for a clean, sharp edge, and its resilient core resists dents. Low pH factor helps protect artwork from discoloring over time. CFC-free.
Black foam board has the same characteristics as the white foam board, but each sheet has a black surface and a black core.