How can I safely use a blender with hot liquids?
It can be a bit tricky, a couple of tips:


Use a lot less liquid in the blender than normal. Do batches if needed.
Vent the lid so the steam can escape. A lot of lids have a center piece that can come out. Using less liquid will stop stuff coming out the top. 
As Ocaasi suggests, you can cover the open lid with a kitchen towel as you start to insure there's no spray. 
Start the blender slow and then speed it up. This may not work depending on how fast your slowest setting is. But once the vortex gets going, the liquid won't splash. It's only when the blades start that you have that issue. Slower start speed means less splash (a vita-mix can start very slowly and have no splash at all, great for hot liquids but the price tag is a bit high).