I am making this new gluten free brownie mix and am wondering if I can substitute the Baker's chocolate squares called for in the recipe, with 70% or higher cocoa bars that I keep on hand. Are there any characteristics that the Baker's chocolate has that a bar wouldn't or couldn't be worked around?
I would say yes. However, depending on what kind of squares it calls for it may change the taste of the brownie or how rich the chocolate flavor is. 70% cocoa bars would be more similar to what Baker's Chocolate sells as Bittersweet (67% cacao) so if the recipe is calling for Unsweetened squares (100% cacao) then you'll have less chocolate flavor (and perhaps a slightly sweeter brownie). If the recipe is calling for Semi-Sweet squares (54% cacao) then you'll have more chocolate flavor (and perhaps a slightly less sweet brownie).
I always use 'normal' chocolate bars in place of Baker's chocolate. The important thing to remember is that you use good quality chocolate with a high percentage of cacao.
That said, there are different kinds of Baker's chocolate, so it is important that you use chocolate that is similar. For example, if a recipe calls for Baker's chocolate with a high percentage of cacao, I would really substitute it for a chocolate bar with high percentage of cacao. However, if they use it as chocolate chucks in the recipe, you can take chocolate with the same amount of cacao (or chocolate with less cacao if that is the taste you prefer).
Every chocolate tastes slightly different, so maybe you have to make the recipe a couple of time before you have the exact flavor you want. High percentage chocolate bars are cheaper and taste very good, so using them will give you a delicious result.