I tried that mission three times...
Three ****ing times...
At one time it hit zero and jumped, after that I googled and found out it will survive no matter what you do, like that other big bad ship when you meet it

Granted, the dialouge could be more like "You drove them of, good job" instead of "Buhu, you failed, it will hound us again", but canon seems to be a thing here, after all Abaddon the Failure will take two Blackstones with him at the end of the war

So, about the discussion Nova Cannon or Missile launcher?
On my first attem I used my Retribution, a Mars and some other Cruiser.
The Blackstone warped away fearly early and had crippled the Mars.
Second attempt I switched to an all out missile attack pattern, I even brought these missile escorts.
A piece of cake.
The Blackstone moves straight at you, so if you line up and launch spread after spread of melta torpedos, turn and burn, you'll get this mission over in not time.
Unfortunately these dumbass AI allys usually end up in my line of fire.
So in battle against a Blackstone or later the Planetkiller, missile are the way to go, they are slow and unable to evade and with a bit off maneuvering, you can kill them without getting a single scratch

And about the AI...
Had a breakthrough mission with my Astartes buddies.
A Strike Cruiser, two Swords and a Firestorm on his side and I brought my big R along... it is a bad joke, you try to support him and that Strikecruiser always runs off, so my big guns cannot support him, the point limit of 300 is not helping either.
Addendum: The Eldar will launch no new attacks in the Sektor, or so I was told.
Icannot emphasis the toughness of a Battleship enough.
I took two Battlecruiser and my Retibution into said battle, add to this an Eldar BB, a Cruiser and several escorts against Orks.
Once again the AI charges ahead, resulting in several escort being ran through by the opposing Ork ships.
While I picked my targets, my Dominator was slowly picked apart, it lost engines, reactor, got an Hullbreach and so on, but at that point, the Eldar had escaped and only their badly damaged BB.
So I was up against an Ork BB and two cruisers.
Since my Gothic has only a range of 6km with its braodside guns, I had my whole fleet set at 6km engagement range.
With the Dominator safely fled(and of course lost in the warp), I set up the range of the big R to 12km and shredded the orks, supported by the Gothic.
The big R was badly damaged herself, reactor gone and an hullbreach as well. So I kept her far away and let the Gothic dive in in, when the microwarp was ready, fired, and warped out.
The AI, braindead stupid chased after the Gothic, so I could repair the big R up to AFAIK 75% and delieverd the killing blow.
It is simply outstanding how much damage this beast can take, only to shred opposing ships.
Right now I choose to run her with Firestorms as escorts, sniping away enemy shields, launching a spread and then thunder the broadsides
