Likely the bitterness has nothing to do with this particular combination of ingredients. Rather, the blending process can break down the structures of each of the ingredients far more thoroughly then chopping and allows bitter compounds to leach out and saturate the overall mixture. Garlic in particular can be bitter if it's been pulverized, but it gets milder as it cooks. Did you notice that the bitterness ruined the dish after it was cooked?
Also, is there a reason to blend these ingredients? It's often preferable to chop ingredients by hand, or even use a food processor.
Also, I'd warn against cooking all of these ingredients at the same time anyway. They all cook at slightly different rates. For example, if you put onions and garlic in the pan at the same time, the garlic will burn long before the onions are done. EDIT: The warning against cooking them at the same time only applies if you are indeed intending to sweat or sauté them. If they are buffered with a liquid, then they won't burn. However, some of the ingredients might get mushy or even dissolve completely before the other ingredients are finished.