If it's overoaked, it will mellow out with some time. But it's still better to not overdo it in the beginning. Add your oak, and then taste weekly. When it's enough, or even just a tiny bit more than enough, rack off of the oak.
Oak is really interesting because different oaks give different flavors. As an example, a light toasted American oak with give flavors of vanilla with a hint of sweetness, but more aggressive oak flavor; while a dark toasted Hungarian oak is deeper, almost like cinnamon but not such an aggressive oak flavor. French oak chips are available also.
I'd probably go with a French medium oak chip if you're unsure if something aggressive would be over the top for your mead, especially since blueberry doesn't stand up that well to oaking anyway.