Hey Fawn-
I have to agree with the others that your main problem is probably the canned blueberries. If you want to use canned blueberries in a recipe that calls for fresh or frozen, especially if they are in syrup, I would suggest you rinse them and let them dry, or at least drain off as much syrup as possible. If you don't want to waste the syrup, you could even add a little water to thin it a bit and use it to replace part of the milk.

If you can get frozen blueberries, though, that would really be best.
I don't think that reducing the sugar would have made a big difference, though. You only reduced it by 1/4 cup, which shouldn't have a huge impact on the final product. Quickbreads are pretty forgiving in this respect.
I don't think that the cooking temperature would have made a large impact either. It could have made some impact as to the rising, but not very much in my experience. I guess some people might argue that it makes a difference, but in my opinion, its not a big one. Many quickbread recipe are cooked at varying temperatures between 325-425, and in my experience the main impact this has is just on the cook time. I do wonder, however, that since you cooked these 25 degrees below the recommended temperature, if you shouldn't have cooked them for longer than recommended in the recipe. They do look a little doughy in the picture, but that may be unavoidable since you added the syrup from the blueberries.
In general, when it comes to muffins and quickbreads, unless they call for butter to be creamed, I don't use my mixer. I find it easier to overmix quickbread batters when using the mixer.
The best/easiest way to mix up most quickbread batters, in my experience, is this:
1) Combine the dry ingredients in a large bowl.
2) Combine the wet ingredients in a medium bowl.
3) Make a well in the center of the dry ingredients, and pour in the wet ingredients.
4) Fold together until all the flour is mixed in. (I find a rubber spatula works best for me.)
5) Fold in any additions (such as berries, nuts, chocolate chips, etc.)
6) Pour into tins and bake!
Mixing your batter by hand lets you gauge much more accurately whether it is all mixed together properly, and lets you target little patches of flour, which decreases your chance of overmixing as you might with a mixer. Its also easier to tell if your additions are evenly distributed and you are less likely to damage tender additons like berries. Overall, don't worry too much about overmixing- I know a lot of recipes exclaim "do not overmix!" but truth be told, it is a lot better for a quickbread to be slightly overmixed than it is for it to have clumps of flour in it.
If you're having trouble with your flour being clumpy, you might want to try sifting it first. A lot of people do this for most recipes, saying that it produces a lighter more tender baked good.
HTH!