I'd be surprised if Blizzard didn't have a legal staff that rivals the number of developers they have. Businesses become about court battles it seems once they reach a certain size. From the semiconductor industry, I know that pretty much any argument of infringement usually at least gets heard in court. There is no reason to believe that at least in the US that pretty much any claims Blizzard chooses to make about their IP wouldn't have a very high chance of being upheld.
I'm not taking sides on this issue. I will say that in this case where they provide the API to their product, they kind of have the high ground.