As previously noted, I prefer using an immersion blender to puree my soups. However, I realize that the immersion blender isn’t a common kitchen tool, so I wanted to outline the correct (safe) procedure for pureeing a hot soup in a regular blender:
1. Hot liquids will expand when blended, so be careful not to fill the jar of the blender more than halfway (puree in batches if necessary).
2. To prevent the liquid from spattering, allow the heat to escape: Remove the cap from the hole in the lid, and cover the lid with a dishtowel when blending.
3. Repeat if necessary. Return soup to a clean saucepan to warm (for warm soups, obviously).*
*Source: Everyday Food: Great Food Fast