Pygame和blitting：白色=灰色？

I'm using pygame (1.9.0rc3, though this also happens in 1.8.1) to create a heatmap. To build the heatmap, I use a small, 24-bit 11x11px dot PNG image with a white background and a very low-opacity grey dot that stops exactly at the edges:I'm using pygame (1.9.0rc3, though this also ha