The Hellfire Warlock text says:
At each level, you gain new invocations known, increased damage with eldritch blast, and an increase in invoker level as if you had also gained a level in the warlock class. You do not, however, gain any other benefit a character of that class would have gained.
I have a Rogue 4/ Warlock 8/ Hellfire Warlock 3 so for my eldritch blast damage I consider myself like a Warlock 11 (6d6).
On my next level I plan to take a level of Warlock, but the feature of the 9th of Warlock is just increase of eldritch blast damage.
Do I increase the damage of my eldritch blast again (maybe a little overpowered) or is the 9th level of Warlock in my build lost (no gain)?