When you are using the blender, electricity going through the motor heats it up.
This heat can vaporize oils and other stuff on or near the motor and give this burning or hot smell.
This type of blender is meant to be PULSED and not continuously used, especially for more than 20 or 30 seconds at a time. Be aware of how warm it gets during use so that you don't burn it out. Let it cool between uses.