It smells because of bacteria, and plastic bottle are porous so bacteria will grow and not easily rinse. You have to kill the bacteria:
8-10 oz of boiling water (or just short of boiling)
2 TBSP bleach
Wrap bottle with a towel (water will make it super hot) and shake HARD for 1-2 minutes. Remove cap because pressure will build up - watch out when unscrewing cap - and then put the cap back on and shake again for 1-2 minutes. Pour out the liquid and rinse with cold water.
I did this to the 5 gal water jub we have on the water cooler because some algae-looking stuff was growing around the rim. The bleach and boiling water destroyed it.
If the blender ball was in there while the stink accumulated, do the above with the blender ball inside.