I made these brownies the other day with cacao and unfortunately I was disappointed in the taste of them too.
I suspect the problem may be the dark brown sugar. Most brownie recipes use standard brown sugar. Even before I cooked mine, the mixture tasted very bitter, almost like molasses which I know is what makes the dark brown sugar dark.
While the recipe works well and the texture is good, unfortunately everyone in our family found them to be way too sour. They didn't really have any sweetness at all and just didn't taste like what a brownie should taste like in my opinion. You really needed to have them with ice cream or cream, they just weren't pleasant to eat on their own.
If you don't like sweet things then perhaps you will love this recipe but if you do then I would give this one a miss.