In my experience, there will be some people who simply don't read the content ratings, pass the game to their children, then complain when something pops up that the parents don't like. I tend to stick to Medium maturity for games with mild swearing ("damn", "hell", etc.), blood spatter, and weapons/killing. However I have never had a single complaint about the blood in my games, and there's quite a bit of blood in my last game which has over 400k downloads on Google Play now. That game included a few swear words similar to those mentioned earlier in this post, and that has been reported twice via user comments as far as I know, even with the content rating.
My opinion, rate it medium maturity and add the blood. It might help to add an option to disable blood effects, but I'm willing to bet most won't have any issues with it. If they do, just point them to your content rating as medium will suffice for 99% of the people who care.