Really, the problem is that BLs are cheap because they are supposed to last just 1 turn. Wouldnt really care too much about this 'combo', it would be extremelly vulnerable to PC (and not so cheap, and would still be a duo), but maybe you are right.
Before thinking about restrict the protection to some elements, and even about reduce the protection, is there something we could do? Maybe requiring 2 quanta of that element at a turns end would do it. If the effect of it being destroyed applies in the moment it happens like it uses to be with Eclipse, then it could work.
Or maybe make BL cost 1, and increse its damage a little, lets say, to 6. Just an idea.
Ideas about this?