Zombolini, neither Blackstar nor Hackman are on the main quest line. They're both just side quests. You don't need to complete them.
Hackman is currently not in game due to a programming bug Runewaker is working on fixing. Griffith Blackstar's respawn time is 8 hours between kills, but you may be missing him because he is easily soloable these days.
But it sounds like you're having issues with the quest chains, so can I get some more info? What level are you currently? Have you completed the Return the Glory and Envoy to the Dragons quest chains in Dalanis, Southern and Northern Janost?
You might actually want to go back to Dalanis and check, if you are very low on experience. It's very easy to miss Return the Glory or Envoy to the Dragons.
The other thing that can happen is that there's a bit of a gap in the experience given in those zones, so you may be a little too low to pick up the quests in Limo Desert.
Let me know and I will try and help you figure out what to do next. But you definitely do not need to kill Hackman or Griffith.
**edited to add**
Lol at Inferiority beating me! You're going to give me an Inferiority complex!
Thanks for the informative replies everyone
I've JUST hit level 60
I've picked up the Envoy of Dragons quest line however I've stumbled across another bug in that doesn't let me finish the quest. Yallien doesn't appear. I've got the cloak on and everything, he's even a followable target but when I get there, he's not there.
Where does one pick up the Return to Glory Quest line?
Ultimately my goal is to be able to get into Angren but the guards keep stiff-arming me as soon as I try and enter. Not sure exactly why... Maybe they don't like my clothes?