Blue damsels can be pretty aggressive (as you know). You will want to match it up with another fish that can hold its own against the damsel. Usually a clown would be a good choice. In a 20 gallon tank, it's a tough situation because that tank size is too small for most fish that would be okay with a damsel. A dwarf angel or a sixline wrasse would probably be fine with the damsel, but they need a larger tank than that. You could try a different type of damsel... They may fight though.