What I would have done if I had the option of doing so, was assign a negative value to "blown saves".
I would have made a save worth, say +2, and a blown save worth, say, -5.
You undervalued a save (IMO). I've played in dozens of leagues, using all kinds of scoring methods, and a save has always been double the value of a blown save. (Also wins were double the value of loses) With this scoring system, a middle reliever is just as valuable as an average closer.
