Flameblast is used on bosses
Channel it to stage 10 to deal the most damage and inflict ignite
Once you ignited a boss - use Flammability to refresh the ignite instead of channell it to stage 10 again
(Defiled forces in passive tree)
You still may use Flameblast on bosses multiple times to deal more dps
If you have currency - buy
Vaal Flameblast, it has way more damage than normal Flameblast and being cast instantly