The few blenders I have inspected closely have a "universal" motor which does work properly with a rheostat. The power to that motor is through the controls on the blender. But the controls on the blender may not work properly with the output from any external control device.
Your post cannot be correct on the wattage.
A rheostat is a variable resistor and simply wastes the unused electricity as heat- a crude way to do it. Using a variable transformer directly to the motor, bypassing the original controls, is a much better way. Running the motor at slower speeds risks overheating no matter how the speed control is accomplished.

If it has a universal motor any of the drill/router speed control

Here is a link that might be useful: http://www.amazon.com/MLCS-9410-20-Amp-Router-Control/dp/B001NIK6PC/ref=pd_sim_sbs_k_2

If the blender already has a speed control it is not going to work well with another control feeding it.

Just set the blender at MAX speed and it should be ok.

There is proper laboratory equipment that will do what you need. Check for paddle mixers and homogenizers of various types. You can probably find a used one for a good price if you shop around. You may well get lucky on ebay. I get some lab equipment there.