We got around to trying it about 3 months ago (by "we" I mean it was my idea, but my husband ended up doing the baking). You turn a muffin mix into waffle batter by simply following the muffin mix instructions then adding some water. Turns out we didn't need to add water, the consistency was right without adding any.
When it came time to dig-in, we didn't notice any difference or improvement in flavor or texture over regular pancake/waffle mix. So I'd say just stick to the regular mix and add blueberries. But if all you have in the cupboard is muffin mix, and you have a hankering for waffles, go for it!