Can I chop up and freeze mirepoix raw, or do the vegetables have to be blanched first? Thanks!
Yes, you can freeze mirepoix, but it'll be pretty soft once defrosted, as Peter V noted. Onions and celery do not freeze well, though carrots do okay.
All three vegetables, when whole, should be blanched for a few minutes before freezing. For vegetable-specific freezing instructions see the National Center for Home Food Preservation's Freezing Guide.
It's best to use fresh, but blanched for 1-2 minutes then frozen works okay. I do not blanch diced vegetables for more than 2 minutes.
What will you be doing with this mirepoix?
If it's just going to be a basis for a stock, you could probably freeze it without ill effect.
If you're going to be caramelizing it in some butter, and then using it in a final dish, I'm not sure that I would freeze it; the veggies seem likely to get mushy, regardless of whether or not they were blanched first.
If you are planning on using the mirepoix as-is, as in it will be seen on the plate, yes you should blanch first and then freeze as fast as you possibly can. This will help retain texture.
Otherwise, if you're using for stock or whatever, go ahead and freeze.