It’s a bit easier said than done.
If you cast BL early (when engaging), most of the time it will be wasted: half of the team gets crowd controlled, the heal is overheal, Valla has no stacks yet, and so on.
So I find it’s best to cast BL (apart from a nice 5v2 / 5v3 before an objective) when the fight is about midway, preferably (enemy) ults spent. If the fight is going fairly well, it’s nice to add a bit of oomph. Valla, Raynor, Varian will easily shred the enemy while the combined healing from trait and BL will make them almost as tanks. Even if it’s only 1-2 confirmed kills in the end (instead of 0), I find it worth it.
And then there is the 2v5 BL when both of us are on 20% health, that’s super obvious. 