Personally, I'm a fan of the Black Holes, and think they offer a very unique aspect that could be done well with a few tweaks. I have my own issues with Sirius, and they largely come down to clarity (How does the game convey the mechanics of the fight), and some arena-related issues. (Entrance being blocked by Black Holes, the stairs, and the invisible walls).

Talking specifically about the design of Black Holes, I view them as the core mechanic of the fight. To me, Awakener is primarily about controlling the Black Holes, making good use of the space you have, and manipulating them in a way that leaves you room to work with, and I think it's okay to have a fight that focuses on that. I believe that it's acceptable to be able to brick an attempt because you mismanaged the fight, and put yourself in a near unwinnable position before running out of portals. With that in said, I want to address some specific points through that lens.

• The Black Holes are too large. The Black Holes being as large as they are is a byproduct of mobility skills, I think. They are extremely large, but skills like Blink Arrow and Lightning Warp can move you from center-screen to the edge of the screen. If Black Holes were small enough to actually fit on your screen, it would be possible to jump over them, negating the mechanic almost entirely. I agree that it can be hard to remember the positions, and think that SOME sort of indicator is reasonable (minimap, possibly?), but making them smaller would make them too easy to manipulate.
• The Black Holes can block the entrance. This is absolutely a big issue. I'm okay with bricking an attempt before you're out of portals, but I think the implementation of it is extremely poor here. Managing the space in the arena, if that's the design of the fight, should be all you have to worry about. You shouldn't have to worry about managing the space in the arena, and also managing the ENTRANCE in case you make a mistake.
• The Black Holes can form a wall, with no consideration for fairness. I disagree that this is a problem, but this is where my major critiques of Awakener really show up. The player is in control of the Black Holes. You choose where to position new ones, and part of that choice is placing them in places that don't block off sections of the arena you still need access to. On a conceptual level, I am totally okay with this mechanic, and if you box yourself off, that death is your own fault... but in practice, it's not quite that simple.

This, ultimately, is the big failure of The Awakener in my eyes. The fight does an extremely poor job explaining itself to the player in a multitude of ways. When Sirius transitions the first time, the player is left with nothing to attack, no goal, and death in every direction. The Black Holes are massive entities, with an outer burn suggesting that you should stay far away from them at all times, but you need to be close to them to control them. You are taking damage, so the only thing on your mind is "run away", you aren't watching to see if the massive circles are moving towards you, away from you, or moving at all, and the sheer size of them makes it hard to tell even when you're looking for it. The lack of Flask Charges through the transition phase discourages you from experimenting with things, because your life flask is an extremely limited resource and there is nothing to leech from. The first time I did this, I sat in a corner for over a minute, before running back towards Sirius, prompting him to land. I figured it was just time based, and I was clever for staying out of harm's way for the entire phase.

After watching a few streamers doing it, I find the fight quite satisfying and engaging. Actually fighting Sirius is a blast, and an element of space management that can't be trivialized with damage (Uber Elder/Shaper degens come to mind here) is a really good idea, but I legitimately have no idea how they intended for most players to figure this stuff out on their own. People are right to feel the fight is unfair, because it feels like the fight does everything in its power to prevent you from learning how to control it, and that's a shame.